Make rb_scan_args handle keywords more similar to Ruby methods (#2460)

Cfuncs that use rb_scan_args with the : entry suffer similar keyword
argument separation issues that Ruby methods suffer if the cfuncs
accept optional or variable arguments.

This makes the following changes to : handling.

* Treats as **kw, prompting keyword argument separation warnings
  if called with a positional hash.

* Do not look for an option hash if empty keywords are provided.
  For backwards compatibility, treat an empty keyword splat as a empty
  mandatory positional hash argument, but emit a a warning, as this
  behavior will be removed in Ruby 3.  The argument number check
  needs to be moved lower so it can correctly handle an empty
  positional argument being added.

* If the last argument is nil and it is necessary to treat it as an option
  hash in order to make sure all arguments are processed, continue to
  treat the last argument as the option hash. Emit a warning in this case,
  as this behavior will be removed in Ruby 3.

* If splitting the keyword hash into two hashes, issue a warning, as we
  will not be splitting hashes in Ruby 3.

* If the keyword argument is required to fill a mandatory positional
  argument, continue to do so, but emit a warning as this behavior will
  be going away in Ruby 3.

* If keyword arguments are provided and the last argument is not a hash,
  that indicates something wrong. This can happen if a cfunc is calling
  rb_scan_args multiple times, and providing arguments that were not
  passed to it from Ruby.  Callers need to switch to the new
  rb_scan_args_kw function, which allows passing of whether keywords
  were provided.

This commit fixes all warnings caused by the changes above.

It switches some function calls to *_kw versions with appropriate
kw_splat flags. If delegating arguments, RB_PASS_CALLED_KEYWORDS
is used.  If creating new arguments, RB_PASS_KEYWORDS is used if
the last argument is a hash to be treated as keywords.

In open_key_args in io.c, use rb_scan_args_kw.
In this case, the arguments provided come from another C
function, not Ruby.  The last argument may or may not be a hash,
so we can't set keyword argument mode.  However, if it is a
hash, we don't want to warn when treating it as keywords.

In Ruby files, make sure to appropriately use keyword splats
or literal keywords when calling Cfuncs that now issue keyword
argument separation warnings through rb_scan_args.  Also, make
sure not to pass nil in place of an option hash.

Work around Kernel#warn warnings due to problems in the Rubygems
override of the method.  There is an open pull request to fix
these issues in Rubygems, but part of the Rubygems tests for
their override fail on ruby-head due to rb_scan_args not
recognizing empty keyword splats, which this commit fixes.

Implementation wise, adding rb_scan_args_kw is kind of a pain,
because rb_scan_args takes a variable number of arguments.
In order to not duplicate all the code, the function internals need
to be split into two functions taking a va_list, and to avoid passing
in a ton of arguments, a single struct argument is used to handle
the variables previously local to the function.

31 files changed, 508 insertions, 179 deletions

diff --git a/class.c b/class.c
index 08e053967a..d0ee7a68fb 100644
--- a/class.c
+++ b/class.c
@@ -1943,85 +1943,172 @@ rb_get_kwargs(VALUE keyword_hash, const ID *table, int required, int optional, V
 #undef extract_kwarg
 }
 
-#undef rb_scan_args
-int
-rb_scan_args(int argc, const VALUE *argv, const char *fmt, ...)
+struct rb_scan_args_t {
+    int argc;
+    const VALUE *argv;
+    va_list vargs;
+    int f_var;
+    int f_hash;
+    int f_block;
+    int n_lead;
+    int n_opt;
+    int n_trail;
+    int n_mand;
+    int argi;
+    int last_idx;
+    VALUE hash;
+    VALUE last_hash;
+    VALUE *tmp_buffer;
+};
+
+static void
+rb_scan_args_parse(int kw_flag, int argc, const VALUE *argv, const char *fmt, struct rb_scan_args_t *arg)
 {
-    int i;
     const char *p = fmt;
-    VALUE *var;
-    va_list vargs;
-    int f_var = 0, f_hash = 0, f_block = 0;
-    int n_lead = 0, n_opt = 0, n_trail = 0, n_mand;
-    int argi = 0, last_idx = -1;
-    VALUE hash = Qnil, last_hash = 0;
+    VALUE *tmp_buffer = arg->tmp_buffer;
+    int keyword_given = 0;
+    int empty_keyword_given = 0;
+    int last_hash_keyword = 0;
+
+    memset(arg, 0, sizeof(*arg));
+    arg->last_idx = -1;
+    arg->hash = Qnil;
+
+    switch(kw_flag) {
+      case RB_SCAN_ARGS_PASS_CALLED_KEYWORDS:
+        if(!(keyword_given = rb_keyword_given_p())) {
+            empty_keyword_given = rb_empty_keyword_given_p();
+        }
+        break;
+      case RB_SCAN_ARGS_KEYWORDS:
+        keyword_given = 1;
+        break;
+      case RB_SCAN_ARGS_EMPTY_KEYWORDS:
+        empty_keyword_given = 1;
+        break;
+      case RB_SCAN_ARGS_LAST_HASH_KEYWORDS:
+        last_hash_keyword = 1;
+        break;
+    }
 
     if (ISDIGIT(*p)) {
-	n_lead = *p - '0';
+        arg->n_lead = *p - '0';
 	p++;
 	if (ISDIGIT(*p)) {
-	    n_opt = *p - '0';
+            arg->n_opt = *p - '0';
 	    p++;
 	}
     }
     if (*p == '*') {
-	f_var = 1;
+        arg->f_var = 1;
 	p++;
     }
     if (ISDIGIT(*p)) {
-	n_trail = *p - '0';
+        arg->n_trail = *p - '0';
 	p++;
     }
     if (*p == ':') {
-	f_hash = 1;
+        arg->f_hash = 1;
 	p++;
     }
     if (*p == '&') {
-	f_block = 1;
+        arg->f_block = 1;
 	p++;
     }
     if (*p != '\0') {
 	rb_fatal("bad scan arg format: %s", fmt);
     }
-    n_mand = n_lead + n_trail;
+    arg->n_mand = arg->n_lead + arg->n_trail;
 
-    if (argc < n_mand)
-	goto argc_error;
+    /* capture an option hash - phase 1: pop */
+    /* Ignore final positional hash if empty keywords given */
+    if (argc > 0 && !(arg->f_hash && empty_keyword_given)) {
+        VALUE last = argv[argc - 1];
+
+        if (arg->f_hash && arg->n_mand < argc) {
+            if (keyword_given) {
+                if (!RB_TYPE_P(last, T_HASH)) {
+                    rb_warn("Keyword flag set when calling rb_scan_args, but last entry is not a hash");
+                }
+                else {
+                    arg->hash = last;
+                }
+            }
+            else if (NIL_P(last)) {
+                /* For backwards compatibility, nil is taken as an empty
+                   option hash only if it is not ambiguous; i.e. '*' is
+                   not specified and arguments are given more than sufficient.
+                   This will be removed in Ruby 3. */
+                if (!arg->f_var && arg->n_mand + arg->n_opt < argc) {
+                    rb_warn("The last argument is nil, treating as empty keywords");
+                    argc--;
+                }
+            }
+            else {
+                arg->hash = rb_check_hash_type(last);
+            }
+
+            /* Ruby 3: Remove if branch, as it will not attempt to split hashes */
+            if (!NIL_P(arg->hash)) {
+                VALUE opts = rb_extract_keywords(&arg->hash);
+
+                if (!(arg->last_hash = arg->hash)) {
+                    if (!keyword_given && !last_hash_keyword) {
+                        /* Warn if treating positional as keyword, as in Ruby 3,
+                           this will be an error */
+                        rb_warn("The last argument is used as the keyword parameter");
+                    }
+                    argc--;
+                }
+                else {
+                    /* Warn if splitting either positional hash to keywords or keywords
+                       to positional hash, as in Ruby 3, no splitting will be done */
+                    rb_warn("The last argument is split into positional and keyword parameters");
+                    arg->last_idx = argc - 1;
+                }
+                arg->hash = opts ? opts : Qnil;
+            }
+        }
+        else if (arg->f_hash && keyword_given && arg->n_mand == argc) {
+            /* Warn if treating keywords as positional, as in Ruby 3, this will be an error */
+            rb_warn("The keyword argument is passed as the last hash parameter");
+        }
+    }
+    if (arg->f_hash && arg->n_mand == argc+1 && empty_keyword_given) {
+        VALUE *ptr = rb_alloc_tmp_buffer2(tmp_buffer, argc+1, sizeof(VALUE));
+        memcpy(ptr, argv, sizeof(VALUE)*argc);
+        ptr[argc] = rb_hash_new();
+        argc++;
+        *(&argv) = ptr;
+        rb_warn("The keyword argument is passed as the last hash parameter");
+    }
 
-    va_start(vargs, fmt);
+    arg->argc = argc;
+    arg->argv = argv;
+}
 
-    /* capture an option hash - phase 1: pop */
-    if (f_hash && n_mand < argc) {
-	VALUE last = argv[argc - 1];
-
-	if (NIL_P(last)) {
-	    /* nil is taken as an empty option hash only if it is not
-	       ambiguous; i.e. '*' is not specified and arguments are
-	       given more than sufficient */
-	    if (!f_var && n_mand + n_opt < argc)
-		argc--;
-	}
-	else {
-	    hash = rb_check_hash_type(last);
-	    if (!NIL_P(hash)) {
-		VALUE opts = rb_extract_keywords(&hash);
-		if (!(last_hash = hash)) argc--;
-		else last_idx = argc - 1;
-		hash = opts ? opts : Qnil;
-	    }
-	}
+static int
+rb_scan_args_assign(struct rb_scan_args_t *arg, va_list vargs)
+{
+    int argi = 0;
+    int i;
+    VALUE *var;
+
+    if (arg->argc < arg->n_mand) {
+        return 1;
     }
+
     /* capture leading mandatory arguments */
-    for (i = n_lead; i-- > 0; ) {
+    for (i = arg->n_lead; i-- > 0; ) {
 	var = va_arg(vargs, VALUE *);
-	if (var) *var = (argi == last_idx) ? last_hash : argv[argi];
+        if (var) *var = (argi == arg->last_idx) ? arg->last_hash : arg->argv[argi];
 	argi++;
     }
     /* capture optional arguments */
-    for (i = n_opt; i-- > 0; ) {
+    for (i = arg->n_opt; i-- > 0; ) {
 	var = va_arg(vargs, VALUE *);
-	if (argi < argc - n_trail) {
-	    if (var) *var = (argi == last_idx) ? last_hash : argv[argi];
+        if (argi < arg->argc - arg->n_trail) {
+            if (var) *var = (argi == arg->last_idx) ? arg->last_hash : arg->argv[argi];
 	    argi++;
 	}
 	else {
@@ -2029,15 +2116,15 @@ rb_scan_args(int argc, const VALUE *argv, const char *fmt, ...)
 	}
     }
     /* capture variable length arguments */
-    if (f_var) {
-	int n_var = argc - argi - n_trail;
+    if (arg->f_var) {
+        int n_var = arg->argc - argi - arg->n_trail;
 
 	var = va_arg(vargs, VALUE *);
 	if (0 < n_var) {
 	    if (var) {
-		int f_last = (last_idx + 1 == argc - n_trail);
-		*var = rb_ary_new4(n_var-f_last, &argv[argi]);
-		if (f_last) rb_ary_push(*var, last_hash);
+                int f_last = (arg->last_idx + 1 == arg->argc - arg->n_trail);
+                *var = rb_ary_new4(n_var - f_last, &arg->argv[argi]);
+                if (f_last) rb_ary_push(*var, arg->last_hash);
 	    }
 	    argi += n_var;
 	}
@@ -2046,18 +2133,18 @@ rb_scan_args(int argc, const VALUE *argv, const char *fmt, ...)
 	}
     }
     /* capture trailing mandatory arguments */
-    for (i = n_trail; i-- > 0; ) {
+    for (i = arg->n_trail; i-- > 0; ) {
 	var = va_arg(vargs, VALUE *);
-	if (var) *var = (argi == last_idx) ? last_hash : argv[argi];
+        if (var) *var = (argi == arg->last_idx) ? arg->last_hash : arg->argv[argi];
 	argi++;
     }
     /* capture an option hash - phase 2: assignment */
-    if (f_hash) {
+    if (arg->f_hash) {
 	var = va_arg(vargs, VALUE *);
-	if (var) *var = hash;
+        if (var) *var = arg->hash;
     }
     /* capture iterator block */
-    if (f_block) {
+    if (arg->f_block) {
 	var = va_arg(vargs, VALUE *);
 	if (rb_block_given_p()) {
 	    *var = rb_block_proc();
@@ -2066,14 +2153,53 @@ rb_scan_args(int argc, const VALUE *argv, const char *fmt, ...)
 	    *var = Qnil;
 	}
     }
-    va_end(vargs);
 
-    if (argi < argc) {
-      argc_error:
-	rb_error_arity(argc, n_mand, f_var ? UNLIMITED_ARGUMENTS : n_mand + n_opt);
+    if (argi < arg->argc) return 1;
+
+    return 0;
+}
+
+#undef rb_scan_args
+int
+rb_scan_args(int argc, const VALUE *argv, const char *fmt, ...)
+{
+    int error;
+    va_list vargs;
+    VALUE tmp_buffer = 0;
+    struct rb_scan_args_t arg;
+    arg.tmp_buffer = &tmp_buffer;
+    rb_scan_args_parse(RB_SCAN_ARGS_PASS_CALLED_KEYWORDS, argc, argv, fmt, &arg);
+    va_start(vargs,fmt);
+    error = rb_scan_args_assign(&arg, vargs);
+    va_end(vargs);
+    if (tmp_buffer) {
+        rb_free_tmp_buffer(&tmp_buffer);
     }
+    if (error) {
+        rb_error_arity(arg.argc, arg.n_mand, arg.f_var ? UNLIMITED_ARGUMENTS : arg.n_mand + arg.n_opt);
+    }
+    return arg.argc;
+}
 
-    return argc;
+int
+rb_scan_args_kw(int kw_flag, int argc, const VALUE *argv, const char *fmt, ...)
+{
+    int error;
+    va_list vargs;
+    VALUE tmp_buffer = 0;
+    struct rb_scan_args_t arg;
+    arg.tmp_buffer = &tmp_buffer;
+    rb_scan_args_parse(kw_flag, argc, argv, fmt, &arg);
+    va_start(vargs,fmt);
+    error = rb_scan_args_assign(&arg, vargs);
+    va_end(vargs);
+    if (tmp_buffer) {
+        rb_free_tmp_buffer(&tmp_buffer);
+    }
+    if (error) {
+        rb_error_arity(arg.argc, arg.n_mand, arg.f_var ? UNLIMITED_ARGUMENTS : arg.n_mand + arg.n_opt);
+    }
+    return arg.argc;
 }
 
 int
diff --git a/dir.c b/dir.c
index f5cc42d99f..6d1f501927 100644
--- a/dir.c
+++ b/dir.c
@@ -2900,7 +2900,7 @@ dir_s_glob(int argc, VALUE *argv, VALUE obj)
 static VALUE
 dir_open_dir(int argc, VALUE *argv)
 {
-    VALUE dir = rb_funcallv(rb_cDir, rb_intern("open"), argc, argv);
+    VALUE dir = rb_funcallv_kw(rb_cDir, rb_intern("open"), argc, argv, RB_PASS_CALLED_KEYWORDS);
 
     rb_check_typeddata(dir, &dir_data_type);
     return dir;
diff --git a/error.c b/error.c
index 87357a4ebe..bef5090011 100644
--- a/error.c
+++ b/error.c
@@ -341,7 +341,7 @@ rb_warn_m(int argc, VALUE *argv, VALUE exc)
     VALUE opts, location = Qnil;
 
     if (!NIL_P(ruby_verbose) && argc > 0 &&
-	    (argc = rb_scan_args(argc, argv, "*:", NULL, &opts)) > 0) {
+            (argc = rb_scan_args(argc, argv, "*:", NULL, &opts)) > 0) {
 	VALUE str = argv[0], uplevel = Qnil;
 	if (!NIL_P(opts)) {
 	    static ID kwds[1];
@@ -1595,7 +1595,7 @@ nometh_err_initialize(int argc, VALUE *argv, VALUE self)
     priv = (argc > 3) && (--argc, RTEST(argv[argc]));
     args = (argc > 2) ? argv[--argc] : Qnil;
     if (!NIL_P(options)) argv[argc++] = options;
-    rb_call_super(argc, argv);
+    rb_call_super_kw(argc, argv, RB_PASS_CALLED_KEYWORDS);
     return nometh_err_init_attr(self, args, priv);
 }
 
diff --git a/ext/-test-/scan_args/scan_args.c b/ext/-test-/scan_args/scan_args.c
index dca353f643..9c374da66f 100644
--- a/ext/-test-/scan_args/scan_args.c
+++ b/ext/-test-/scan_args/scan_args.c
@@ -250,6 +250,33 @@ scan_args_lead_opt_var_trail_hash(int argc, VALUE *argv, VALUE self)
     return rb_ary_new_from_values(numberof(args), args);
 }
 
+static VALUE
+scan_args_k_lead_opt_hash(int argc, VALUE *argv, VALUE self)
+{
+    VALUE args[4];
+    int n = rb_scan_args_kw(RB_SCAN_ARGS_KEYWORDS, argc, argv, "11:", args+1, args+2, args+3);
+    args[0] = INT2NUM(n);
+    return rb_ary_new_from_values(numberof(args), args);
+}
+
+static VALUE
+scan_args_e_lead_opt_hash(int argc, VALUE *argv, VALUE self)
+{
+    VALUE args[4];
+    int n = rb_scan_args_kw(RB_SCAN_ARGS_EMPTY_KEYWORDS, argc, argv, "11:", args+1, args+2, args+3);
+    args[0] = INT2NUM(n);
+    return rb_ary_new_from_values(numberof(args), args);
+}
+
+static VALUE
+scan_args_n_lead_opt_hash(int argc, VALUE *argv, VALUE self)
+{
+    VALUE args[4];
+    int n = rb_scan_args_kw(RB_SCAN_ARGS_LAST_HASH_KEYWORDS, argc, argv, "11:", args+1, args+2, args+3);
+    args[0] = INT2NUM(n);
+    return rb_ary_new_from_values(numberof(args), args);
+}
+
 void
 Init_scan_args(void)
 {
@@ -282,5 +309,7 @@ Init_scan_args(void)
     rb_define_singleton_method(module, "lead_var_trail_hash", scan_args_lead_var_trail_hash, -1);
     rb_define_singleton_method(module, "opt_var_trail_hash", scan_args_opt_var_trail_hash, -1);
     rb_define_singleton_method(module, "lead_opt_var_trail_hash", scan_args_lead_opt_var_trail_hash, -1);
+    rb_define_singleton_method(module, "k_lead_opt_hash", scan_args_k_lead_opt_hash, -1);
+    rb_define_singleton_method(module, "e_lead_opt_hash", scan_args_e_lead_opt_hash, -1);
+    rb_define_singleton_method(module, "n_lead_opt_hash", scan_args_n_lead_opt_hash, -1);
 }
-
diff --git a/ext/json/lib/json/common.rb b/ext/json/lib/json/common.rb
index 7cc852916c..3be9fd8dc5 100644
--- a/ext/json/lib/json/common.rb
+++ b/ext/json/lib/json/common.rb
@@ -153,7 +153,7 @@ module JSON
   # * *object_class*: Defaults to Hash
   # * *array_class*: Defaults to Array
   def parse(source, opts = {})
-    Parser.new(source, opts).parse
+    Parser.new(source, **(opts||{})).parse
   end
 
   # Parse the JSON document _source_ into a Ruby data structure and return it.
@@ -176,7 +176,7 @@ module JSON
       :max_nesting  => false,
       :allow_nan    => true
     }.merge(opts)
-    Parser.new(source, opts).parse
+    Parser.new(source, **(opts||{})).parse
   end
 
   # Generate a JSON document from the Ruby data structure _obj_ and return
diff --git a/ext/json/parser/parser.c b/ext/json/parser/parser.c
index 6f0d31c2eb..2d13bccf7e 100644
--- a/ext/json/parser/parser.c
+++ b/ext/json/parser/parser.c
@@ -1835,7 +1835,7 @@ static VALUE cParser_initialize(int argc, VALUE *argv, VALUE self)
     } else {
         json->max_nesting = 100;
         json->allow_nan = 0;
-        json->create_additions = 1;
+        json->create_additions = 0;
         json->create_id = rb_funcall(mJSON, i_create_id, 0);
         json->object_class = Qnil;
         json->array_class = Qnil;
diff --git a/ext/pathname/pathname.c b/ext/pathname/pathname.c
index 1cf0f90b9f..022a87699b 100644
--- a/ext/pathname/pathname.c
+++ b/ext/pathname/pathname.c
@@ -393,7 +393,7 @@ path_read(int argc, VALUE *argv, VALUE self)
 
     args[0] = get_strpath(self);
     n = rb_scan_args(argc, argv, "03", &args[1], &args[2], &args[3]);
-    return rb_funcallv(rb_cFile, id_read, 1+n, args);
+    return rb_funcallv_kw(rb_cFile, id_read, 1+n, args, RB_PASS_CALLED_KEYWORDS);
 }
 
 /*
@@ -1097,12 +1097,12 @@ path_s_glob(int argc, VALUE *argv, VALUE klass)
 
     n = rb_scan_args(argc, argv, "12", &args[0], &args[1], &args[2]);
     if (rb_block_given_p()) {
-        return rb_block_call(rb_cDir, id_glob, n, args, s_glob_i, klass);
+        return rb_block_call_kw(rb_cDir, id_glob, n, args, s_glob_i, klass, RB_PASS_CALLED_KEYWORDS);
     }
     else {
         VALUE ary;
         long i;
-        ary = rb_funcallv(rb_cDir, id_glob, n, args);
+        ary = rb_funcallv_kw(rb_cDir, id_glob, n, args, RB_PASS_CALLED_KEYWORDS);
         ary = rb_convert_type(ary, T_ARRAY, "Array", "to_ary");
         for (i = 0; i < RARRAY_LEN(ary); i++) {
             VALUE elt = RARRAY_AREF(ary, i);
@@ -1145,12 +1145,12 @@ path_glob(int argc, VALUE *argv, VALUE self)
     n = 3;
 
     if (rb_block_given_p()) {
-        return rb_block_call(rb_cDir, id_glob, n, args, glob_i, self);
+        return rb_block_call_kw(rb_cDir, id_glob, n, args, glob_i, self, RB_PASS_KEYWORDS);
     }
     else {
         VALUE ary;
         long i;
-        ary = rb_funcallv(rb_cDir, id_glob, n, args);
+        ary = rb_funcallv_kw(rb_cDir, id_glob, n, args, RB_PASS_KEYWORDS);
         ary = rb_convert_type(ary, T_ARRAY, "Array", "to_ary");
         for (i = 0; i < RARRAY_LEN(ary); i++) {
             VALUE elt = RARRAY_AREF(ary, i);
diff --git a/ext/stringio/stringio.c b/ext/stringio/stringio.c
index 4a81e84183..ab533a03fb 100644
--- a/ext/stringio/stringio.c
+++ b/ext/stringio/stringio.c
@@ -385,7 +385,7 @@ strio_finalize(VALUE self)
 static VALUE
 strio_s_open(int argc, VALUE *argv, VALUE klass)
 {
-    VALUE obj = rb_class_new_instance(argc, argv, klass);
+    VALUE obj = rb_class_new_instance_kw(argc, argv, klass, RB_PASS_CALLED_KEYWORDS);
     if (!rb_block_given_p()) return obj;
     return rb_ensure(rb_yield, obj, strio_finalize, obj);
 }
@@ -400,7 +400,7 @@ strio_s_new(int argc, VALUE *argv, VALUE klass)
 	rb_warn("%"PRIsVALUE"::new() does not take block; use %"PRIsVALUE"::open() instead",
 		cname, cname);
     }
-    return rb_class_new_instance(argc, argv, klass);
+    return rb_class_new_instance_kw(argc, argv, klass, RB_PASS_CALLED_KEYWORDS);
 }
 
 /*
diff --git a/ext/zlib/zlib.c b/ext/zlib/zlib.c
index afd761f1c1..22b4d0fb8d 100644
--- a/ext/zlib/zlib.c
+++ b/ext/zlib/zlib.c
@@ -3045,7 +3045,7 @@ static VALUE
 new_wrap(VALUE tmp)
 {
     new_wrap_arg_t *arg = (new_wrap_arg_t *)tmp;
-    return rb_class_new_instance(arg->argc, arg->argv, arg->klass);
+    return rb_class_new_instance_kw(arg->argc, arg->argv, arg->klass, RB_PASS_CALLED_KEYWORDS);
 }
 
 static VALUE
@@ -3078,7 +3078,7 @@ gzfile_wrap(int argc, VALUE *argv, VALUE klass, int close_io_on_error)
 	}
     }
     else {
-	obj = rb_class_new_instance(argc, argv, klass);
+	obj = rb_class_new_instance_kw(argc, argv, klass, RB_PASS_CALLED_KEYWORDS);
     }
 
     if (rb_block_given_p()) {
diff --git a/include/ruby/ruby.h b/include/ruby/ruby.h
index 71a1d2e83d..6f7708009a 100644
--- a/include/ruby/ruby.h
+++ b/include/ruby/ruby.h
@@ -1899,6 +1899,11 @@ VALUE rb_funcall_passing_block(VALUE, ID, int, const VALUE*);
 VALUE rb_funcall_with_block(VALUE, ID, int, const VALUE*, VALUE);
 VALUE rb_funcall_with_block_kw(VALUE, ID, int, const VALUE*, VALUE, int);
 int rb_scan_args(int, const VALUE*, const char*, ...);
+#define RB_SCAN_ARGS_PASS_CALLED_KEYWORDS 0
+#define RB_SCAN_ARGS_KEYWORDS 1
+#define RB_SCAN_ARGS_EMPTY_KEYWORDS 2
+#define RB_SCAN_ARGS_LAST_HASH_KEYWORDS 3
+int rb_scan_args_kw(int, int, const VALUE*, const char*, ...);
 VALUE rb_call_super(int, const VALUE*);
 VALUE rb_call_super_kw(int, const VALUE*, int);
 VALUE rb_current_receiver(void);
@@ -2324,6 +2329,9 @@ unsigned long ruby_strtoul(const char *str, char **endptr, int base);
 PRINTF_ARGS(int ruby_snprintf(char *str, size_t n, char const *fmt, ...), 3, 4);
 int ruby_vsnprintf(char *str, size_t n, char const *fmt, va_list ap);
 
+/* -- Remove In 3.0, Only public for rb_scan_args optimized version -- */
+int rb_empty_keyword_given_p(void);
+
 #if defined(HAVE_BUILTIN___BUILTIN_CHOOSE_EXPR_CONSTANT_P) && defined(HAVE_VA_ARGS_MACRO) && defined(__OPTIMIZE__)
 # define rb_scan_args(argc,argvp,fmt,...) \
     __builtin_choose_expr(__builtin_constant_p(fmt), \
@@ -2519,30 +2527,81 @@ rb_scan_args_set(int argc, const VALUE *argv,
     int i, argi = 0, vari = 0, last_idx = -1;
     VALUE *var, hash = Qnil, last_hash = 0;
     const int n_mand = n_lead + n_trail;
+    int keyword_given = rb_keyword_given_p();
+    int empty_keyword_given = 0;
+    VALUE tmp_buffer = 0;
+
+    if (!keyword_given) {
+        empty_keyword_given = rb_empty_keyword_given_p();
+    }
 
     /* capture an option hash - phase 1: pop */
-    if (f_hash && n_mand < argc) {
-	VALUE last = argv[argc - 1];
-
-	if (RB_NIL_P(last)) {
-	    /* nil is taken as an empty option hash only if it is not
-	       ambiguous; i.e. '*' is not specified and arguments are
-	       given more than sufficient */
-	    if (!f_var && n_mand + n_opt < argc)
-		argc--;
-	}
-	else {
-	    hash = rb_check_hash_type(last);
-	    if (!RB_NIL_P(hash)) {
-		VALUE opts = rb_extract_keywords(&hash);
-		if (!(last_hash = hash)) argc--;
-		else last_idx = argc - 1;
-		hash = opts ? opts : Qnil;
-	    }
-	}
+    /* Ignore final positional hash if empty keywords given */
+    if (argc > 0 && !(f_hash && empty_keyword_given)) {
+        VALUE last = argv[argc - 1];
+
+        if (f_hash && n_mand < argc) {
+            if (keyword_given) {
+                if (!RB_TYPE_P(last, T_HASH)) {
+                    rb_warn("Keyword flag set when calling rb_scan_args, but last entry is not a hash");
+                }
+                else {
+                    hash = last;
+                }
+            }
+            else if (NIL_P(last)) {
+                /* For backwards compatibility, nil is taken as an empty
+                   option hash only if it is not ambiguous; i.e. '*' is
+                   not specified and arguments are given more than sufficient.
+                   This will be removed in Ruby 3. */
+                if (!f_var && n_mand + n_opt < argc) {
+                    rb_warn("The last argument is nil, treating as empty keywords");
+                    argc--;
+                }
+            }
+            else {
+                hash = rb_check_hash_type(last);
+            }
+
+            /* Ruby 3: Remove if branch, as it will not attempt to split hashes */
+            if (!NIL_P(hash)) {
+                VALUE opts = rb_extract_keywords(&hash);
+
+                if (!(last_hash = hash)) {
+                    if (!keyword_given) {
+                        /* Warn if treating positional as keyword, as in Ruby 3,
+                           this will be an error */
+                        rb_warn("The last argument is used as the keyword parameter");
+                    }
+                    argc--;
+                }
+                else {
+                    /* Warn if splitting either positional hash to keywords or keywords
+                       to positional hash, as in Ruby 3, no splitting will be done */
+                    rb_warn("The last argument is split into positional and keyword parameters");
+                    last_idx = argc - 1;
+                }
+                hash = opts ? opts : Qnil;
+            }
+        }
+        else if (f_hash && keyword_given && n_mand == argc) {
+            /* Warn if treating keywords as positional, as in Ruby 3, this will be an error */
+            rb_warn("The keyword argument is passed as the last hash parameter");
+        }
+    }
+    if (f_hash && n_mand == argc+1 && empty_keyword_given) {
+        VALUE *ptr = (VALUE *)rb_alloc_tmp_buffer2(&tmp_buffer, argc+1, sizeof(VALUE));
+        memcpy(ptr, argv, sizeof(VALUE)*argc);
+        ptr[argc] = rb_hash_new();
+        argc++;
+        *(&argv) = ptr;
+        rb_warn("The keyword argument is passed as the last hash parameter");
     }
 
-    rb_check_arity(argc, n_mand, f_var ? UNLIMITED_ARGUMENTS : n_mand + n_opt);
+
+    if (argc < n_mand) {
+        goto argc_error;
+    }
 
     /* capture leading mandatory arguments */
     for (i = n_lead; i-- > 0; ) {
@@ -2600,6 +2659,13 @@ rb_scan_args_set(int argc, const VALUE *argv,
 	}
     }
 
+    if (argi < argc) {
+      argc_error:
+        if(tmp_buffer) rb_free_tmp_buffer(&tmp_buffer);
+        rb_error_arity(argc, n_mand, f_var ? UNLIMITED_ARGUMENTS : n_mand + n_opt);
+    }
+
+    if(tmp_buffer) rb_free_tmp_buffer(&tmp_buffer);
     return argc;
 }
 #endif
diff --git a/internal.h b/internal.h
index b000feee29..bb298d2bfb 100644
--- a/internal.h
+++ b/internal.h
@@ -1552,9 +1552,6 @@ void rb_class_modify_check(VALUE);
 #define id_status ruby_static_id_status
 NORETURN(VALUE rb_f_raise(int argc, VALUE *argv));
 
-/* -- Remove In 3.0 -- */
-int rb_empty_keyword_given_p(void);
-
 /* eval_error.c */
 VALUE rb_get_backtrace(VALUE info);
 
diff --git a/io.c b/io.c
index 2520725814..2de3cebefb 100644
--- a/io.c
+++ b/io.c
@@ -7022,7 +7022,7 @@ rb_open_file(int argc, const VALUE *argv, VALUE io)
 static VALUE
 rb_io_s_open(int argc, VALUE *argv, VALUE klass)
 {
-    VALUE io = rb_class_new_instance(argc, argv, klass);
+    VALUE io = rb_class_new_instance_kw(argc, argv, klass, RB_PASS_CALLED_KEYWORDS);
 
     if (rb_block_given_p()) {
 	return rb_ensure(rb_yield, io, io_close, io);
@@ -7209,7 +7209,7 @@ rb_f_open(int argc, VALUE *argv, VALUE _)
 	}
     }
     if (redirect) {
-	VALUE io = rb_funcallv(argv[0], to_open, argc-1, argv+1);
+        VALUE io = rb_funcallv_kw(argv[0], to_open, argc-1, argv+1, RB_PASS_CALLED_KEYWORDS);
 
 	if (rb_block_given_p()) {
 	    return rb_ensure(rb_yield, io, io_close, io);
@@ -8390,7 +8390,7 @@ rb_io_s_new(int argc, VALUE *argv, VALUE klass)
 	rb_warn("%"PRIsVALUE"::new() does not take block; use %"PRIsVALUE"::open() instead",
 		cname, cname);
     }
-    return rb_class_new_instance(argc, argv, klass);
+    return rb_class_new_instance_kw(argc, argv, klass, RB_PASS_CALLED_KEYWORDS);
 }
 
 
@@ -10373,7 +10373,7 @@ open_key_args(VALUE klass, int argc, VALUE *argv, VALUE opt, struct foreach_arg
 	v = rb_to_array_type(v);
 	n = RARRAY_LENINT(v);
 	rb_check_arity(n, 0, 3); /* rb_io_open */
-	rb_scan_args(n, RARRAY_CONST_PTR(v), "02:", &vmode, &vperm, &opt);
+        rb_scan_args_kw(RB_SCAN_ARGS_LAST_HASH_KEYWORDS, n, RARRAY_CONST_PTR(v), "02:", &vmode, &vperm, &opt);
     }
     arg->io = rb_io_open(klass, path, vmode, vperm, opt);
 }
diff --git a/lib/csv.rb b/lib/csv.rb
index 61702137d0..60dbbcc230 100644
--- a/lib/csv.rb
+++ b/lib/csv.rb
@@ -637,7 +637,7 @@ class CSV
     file_opts = {universal_newline: false}.merge(options)
 
     begin
-      f = File.open(filename, mode, file_opts)
+      f = File.open(filename, mode, **file_opts)
     rescue ArgumentError => e
       raise unless /needs binmode/.match?(e.message) and mode == "r"
       mode = "rb"
diff --git a/lib/fileutils.rb b/lib/fileutils.rb
index cfc6ef1ec8..11321763a5 100644
--- a/lib/fileutils.rb
+++ b/lib/fileutils.rb
@@ -1276,9 +1276,9 @@ module FileUtils
       opts[:encoding] = ::Encoding::UTF_8 if fu_windows?
 
       files = if Dir.respond_to?(:children)
-        Dir.children(path, opts)
+        Dir.children(path, **opts)
       else
-        Dir.entries(path(), opts)
+        Dir.entries(path(), **opts)
            .reject {|n| n == '.' or n == '..' }
       end
 
diff --git a/lib/open-uri.rb b/lib/open-uri.rb
index d9517e4b9e..f4cb536006 100644
--- a/lib/open-uri.rb
+++ b/lib/open-uri.rb
@@ -10,15 +10,15 @@ module Kernel
     alias open_uri_original_open open # :nodoc:
   end
 
-  def open(name, *rest, &block) # :nodoc:
+  def open(name, *rest, **kw, &block) # :nodoc:
     if (name.respond_to?(:open) && !name.respond_to?(:to_path)) ||
        (name.respond_to?(:to_str) &&
         %r{\A[A-Za-z][A-Za-z0-9+\-\.]*://} =~ name &&
         (uri = URI.parse(name)).respond_to?(:open))
       warn('calling URI.open via Kernel#open is deprecated, call URI.open directly', uplevel: 1)
-      URI.open(name, *rest, &block)
+      URI.open(name, *rest, **kw, &block)
     else
-      open_uri_original_open(name, *rest, &block)
+      open_uri_original_open(name, *rest, **kw, &block)
     end
   end
   module_function :open
diff --git a/lib/pstore.rb b/lib/pstore.rb
index 4daa2e003f..1180fd50a0 100644
--- a/lib/pstore.rb
+++ b/lib/pstore.rb
@@ -374,7 +374,7 @@ class PStore
   def open_and_lock_file(filename, read_only)
     if read_only
       begin
-        file = File.new(filename, RD_ACCESS)
+        file = File.new(filename, **RD_ACCESS)
         begin
           file.flock(File::LOCK_SH)
           return file
@@ -386,7 +386,7 @@ class PStore
         return nil
       end
     else
-      file = File.new(filename, RDWR_ACCESS)
+      file = File.new(filename, **RDWR_ACCESS)
       file.flock(File::LOCK_EX)
       return file
     end
@@ -449,7 +449,7 @@ class PStore
 
   def save_data_with_atomic_file_rename_strategy(data, file)
     temp_filename = "#{@filename}.tmp.#{Process.pid}.#{rand 1000000}"
-    temp_file = File.new(temp_filename, WR_ACCESS)
+    temp_file = File.new(temp_filename, **WR_ACCESS)
     begin
       temp_file.flock(File::LOCK_EX)
       temp_file.write(data)
diff --git a/lib/rubygems/core_ext/kernel_warn.rb b/lib/rubygems/core_ext/kernel_warn.rb
index 3e531441ed..6c5b387e88 100755
--- a/lib/rubygems/core_ext/kernel_warn.rb
+++ b/lib/rubygems/core_ext/kernel_warn.rb
@@ -12,9 +12,9 @@ if RUBY_VERSION >= "2.5"
 
     original_warn = method(:warn)
 
-    module_function define_method(:warn) {|*messages, uplevel: nil|
-      unless uplevel
-        return original_warn.call(*messages)
+    module_function define_method(:warn) {|*messages, **kw|
+      unless uplevel = kw[:uplevel]
+        return original_warn.call(*messages, **kw)
       end
 
       # Ensure `uplevel` fits a `long`
@@ -39,7 +39,8 @@ if RUBY_VERSION >= "2.5"
         end
         uplevel = start
       end
-      original_warn.call(*messages, uplevel: uplevel)
+      kw[:uplevel] = uplevel
+      original_warn.call(*messages, **kw)
     }
   end
 end
diff --git a/lib/rubygems/package.rb b/lib/rubygems/package.rb
index b5f83205b3..49cdf98b31 100644
--- a/lib/rubygems/package.rb
+++ b/lib/rubygems/package.rb
@@ -529,11 +529,11 @@ EOM
     when 'metadata' then
       @spec = Gem::Specification.from_yaml entry.read
     when 'metadata.gz' then
-      args = [entry]
-      args << { :external_encoding => Encoding::UTF_8 } if
+      opts = {}
+      opts[:external_encoding] = Encoding::UTF_8 if
         Zlib::GzipReader.method(:wrap).arity != 1
 
-      Zlib::GzipReader.wrap(*args) do |gzio|
+      Zlib::GzipReader.wrap(entry, **opts) do |gzio|
         @spec = Gem::Specification.from_yaml gzio.read
       end
     end
diff --git a/lib/tempfile.rb b/lib/tempfile.rb
index 2bd124abea..f7caf65f0c 100644
--- a/lib/tempfile.rb
+++ b/lib/tempfile.rb
@@ -130,7 +130,7 @@ class Tempfile < DelegateClass(File)
     @mode = mode|File::RDWR|File::CREAT|File::EXCL
     ::Dir::Tmpname.create(basename, tmpdir, **options) do |tmpname, n, opts|
       opts[:perm] = 0600
-      @tmpfile = File.open(tmpname, @mode, opts)
+      @tmpfile = File.open(tmpname, @mode, **opts)
       @opts = opts.freeze
     end
     ObjectSpace.define_finalizer(self, Remover.new(@tmpfile))
@@ -142,7 +142,7 @@ class Tempfile < DelegateClass(File)
   def open
     _close
     mode = @mode & ~(File::CREAT|File::EXCL)
-    @tmpfile = File.open(@tmpfile.path, mode, @opts)
+    @tmpfile = File.open(@tmpfile.path, mode, **@opts)
     __setobj__(@tmpfile)
   end
 
@@ -329,7 +329,7 @@ def Tempfile.create(basename="", tmpdir=nil, mode: 0, **options)
   Dir::Tmpname.create(basename, tmpdir, **options) do |tmpname, n, opts|
     mode |= File::RDWR|File::CREAT|File::EXCL
     opts[:perm] = 0600
-    tmpfile = File.open(tmpname, mode, opts)
+    tmpfile = File.open(tmpname, mode, **opts)
   end
   if block_given?
     begin
diff --git a/rational.c b/rational.c
index 0dfc8abe12..bd71c215ee 100644
--- a/rational.c
+++ b/rational.c
@@ -1543,7 +1543,7 @@ nurat_round_n(int argc, VALUE *argv, VALUE self)
 {
     VALUE opt;
     enum ruby_num_rounding_mode mode = (
-	argc = rb_scan_args(argc, argv, "*:", NULL, &opt),
+        argc = rb_scan_args(argc, argv, "*:", NULL, &opt),
 	rb_num_get_rounding_option(opt));
     VALUE (*round_func)(VALUE) = ROUND_FUNC(mode, nurat_round);
     return f_round_common(argc, argv, self, round_func);
diff --git a/test/-ext-/test_scan_args.rb b/test/-ext-/test_scan_args.rb
index cb2dab5760..949495dd0d 100644
--- a/test/-ext-/test_scan_args.rb
+++ b/test/-ext-/test_scan_args.rb
@@ -93,7 +93,14 @@ class TestScanArgs < Test::Unit::TestCase
     assert_equal([1, "a", nil], Bug::ScanArgs.lead_hash("a"))
     assert_raise(ArgumentError) {Bug::ScanArgs.lead_hash("a", "b")}
     assert_equal([1, "a", {b: 1}], Bug::ScanArgs.lead_hash("a", b: 1))
-    assert_equal([1, {b: 1}, nil], Bug::ScanArgs.lead_hash(b: 1))
+    assert_warn(/The keyword argument is passed as the last hash parameter/) do
+      assert_equal([1, {b: 1}, nil], Bug::ScanArgs.lead_hash(b: 1))
+    end
+    assert_equal([1, {"a"=>0, b: 1}, nil], Bug::ScanArgs.lead_hash({"a"=>0, b: 1}, **{}))
+    assert_raise(ArgumentError) {Bug::ScanArgs.lead_hash(1, {"a"=>0, b: 1}, **{})}
+    assert_warn(/The keyword argument is passed as the last hash parameter/) do
+      assert_equal([1, {}, nil], Bug::ScanArgs.lead_hash(**{}))
+    end
   end
 
   def test_opt_hash
@@ -102,7 +109,10 @@ class TestScanArgs < Test::Unit::TestCase
     assert_equal([0, nil, {b: 1}], Bug::ScanArgs.opt_hash(b: 1))
     assert_equal([1, "a", {b: 1}], Bug::ScanArgs.opt_hash("a", b: 1))
     assert_raise(ArgumentError) {Bug::ScanArgs.opt_hash("a", "b")}
-    assert_equal([1, {"a"=>0}, {b: 1}], Bug::ScanArgs.opt_hash("a"=>0, b: 1))
+    assert_warn(/The last argument is split into positional and keyword parameters/) do
+      assert_equal([1, {"a"=>0}, {b: 1}], Bug::ScanArgs.opt_hash("a"=>0, b: 1))
+    end
+    assert_equal([1, {"a"=>0, b: 1}, nil], Bug::ScanArgs.opt_hash({"a"=>0, b: 1}, **{}))
   end
 
   def test_lead_opt_hash
@@ -110,9 +120,13 @@ class TestScanArgs < Test::Unit::TestCase
     assert_equal([2, "a", "b", nil], Bug::ScanArgs.lead_opt_hash("a", "b"))
     assert_equal([1, "a", nil, {c: 1}], Bug::ScanArgs.lead_opt_hash("a", c: 1))
     assert_equal([2, "a", "b", {c: 1}], Bug::ScanArgs.lead_opt_hash("a", "b", c: 1))
-    assert_equal([1, {c: 1}, nil, nil], Bug::ScanArgs.lead_opt_hash(c: 1))
+    assert_warn(/The keyword argument is passed as the last hash parameter/) do
+      assert_equal([1, {c: 1}, nil, nil], Bug::ScanArgs.lead_opt_hash(c: 1))
+    end
     assert_raise(ArgumentError) {Bug::ScanArgs.lead_opt_hash("a", "b", "c")}
-    assert_equal([2, "a", {"b"=>0}, {c: 1}], Bug::ScanArgs.lead_opt_hash("a", "b"=>0, c: 1))
+    assert_warn(/The last argument is split into positional and keyword parameters/) do
+      assert_equal([2, "a", {"b"=>0}, {c: 1}], Bug::ScanArgs.lead_opt_hash("a", "b"=>0, c: 1))
+    end
   end
 
   def test_var_hash
@@ -120,7 +134,9 @@ class TestScanArgs < Test::Unit::TestCase
     assert_equal([1, ["a"], nil], Bug::ScanArgs.var_hash("a"))
     assert_equal([1, ["a"], {b: 1}], Bug::ScanArgs.var_hash("a", b: 1))
     assert_equal([0, [], {b: 1}], Bug::ScanArgs.var_hash(b: 1))
-    assert_equal([1, [{"a"=>0}], {b: 1}], Bug::ScanArgs.var_hash("a"=>0, b: 1))
+    assert_warn(/The last argument is split into positional and keyword parameters/) do
+      assert_equal([1, [{"a"=>0}], {b: 1}], Bug::ScanArgs.var_hash("a"=>0, b: 1))
+    end
   end
 
   def test_lead_var_hash
@@ -129,9 +145,13 @@ class TestScanArgs < Test::Unit::TestCase
     assert_equal([2, "a", ["b"], nil], Bug::ScanArgs.lead_var_hash("a", "b"))
     assert_equal([2, "a", ["b"], {c: 1}], Bug::ScanArgs.lead_var_hash("a", "b", c: 1))
     assert_equal([1, "a", [], {c: 1}], Bug::ScanArgs.lead_var_hash("a", c: 1))
-    assert_equal([1, {c: 1}, [], nil], Bug::ScanArgs.lead_var_hash(c: 1))
+    assert_warn(/The keyword argument is passed as the last hash parameter/) do
+      assert_equal([1, {c: 1}, [], nil], Bug::ScanArgs.lead_var_hash(c: 1))
+    end
     assert_equal([3, "a", ["b", "c"], nil], Bug::ScanArgs.lead_var_hash("a", "b", "c"))
-    assert_equal([2, "a", [{"b"=>0}], {c: 1}], Bug::ScanArgs.lead_var_hash("a", "b"=>0, c: 1))
+    assert_warn(/The last argument is split into positional and keyword parameters/) do
+      assert_equal([2, "a", [{"b"=>0}], {c: 1}], Bug::ScanArgs.lead_var_hash("a", "b"=>0, c: 1))
+    end
   end
 
   def test_opt_var_hash
@@ -142,7 +162,9 @@ class TestScanArgs < Test::Unit::TestCase
     assert_equal([1, "a", [], {c: 1}], Bug::ScanArgs.opt_var_hash("a", c: 1))
     assert_equal([0, nil, [], {c: 1}], Bug::ScanArgs.opt_var_hash(c: 1))
     assert_equal([3, "a", ["b", "c"], nil], Bug::ScanArgs.opt_var_hash("a", "b", "c"))
-    assert_equal([2, "a", [{"b"=>0}], {c: 1}], Bug::ScanArgs.opt_var_hash("a", "b"=>0, c: 1))
+    assert_warn(/The last argument is split into positional and keyword parameters/) do
+      assert_equal([2, "a", [{"b"=>0}], {c: 1}], Bug::ScanArgs.opt_var_hash("a", "b"=>0, c: 1))
+    end
   end
 
   def test_lead_opt_var_hash
@@ -151,10 +173,14 @@ class TestScanArgs < Test::Unit::TestCase
     assert_equal([2, "a", "b", [], nil], Bug::ScanArgs.lead_opt_var_hash("a", "b"))
     assert_equal([2, "a", "b", [], {c: 1}], Bug::ScanArgs.lead_opt_var_hash("a", "b", c: 1))
     assert_equal([1, "a", nil, [], {c: 1}], Bug::ScanArgs.lead_opt_var_hash("a", c: 1))
-    assert_equal([1, {c: 1}, nil, [], nil], Bug::ScanArgs.lead_opt_var_hash(c: 1))
+    assert_warn(/The keyword argument is passed as the last hash parameter/) do
+      assert_equal([1, {c: 1}, nil, [], nil], Bug::ScanArgs.lead_opt_var_hash(c: 1))
+    end
     assert_equal([3, "a", "b", ["c"], nil], Bug::ScanArgs.lead_opt_var_hash("a", "b", "c"))
     assert_equal([3, "a", "b", ["c"], {d: 1}], Bug::ScanArgs.lead_opt_var_hash("a", "b", "c", d: 1))
-    assert_equal([3, "a", "b", [{"c"=>0}], {d: 1}], Bug::ScanArgs.lead_opt_var_hash("a", "b", "c"=>0, d: 1))
+    assert_warn(/The last argument is split into positional and keyword parameters/) do
+      assert_equal([3, "a", "b", [{"c"=>0}], {d: 1}], Bug::ScanArgs.lead_opt_var_hash("a", "b", "c"=>0, d: 1))
+    end
   end
 
   def test_opt_trail_hash
@@ -163,9 +189,13 @@ class TestScanArgs < Test::Unit::TestCase
     assert_equal([2, "a", "b", nil], Bug::ScanArgs.opt_trail_hash("a", "b"))
     assert_equal([1, nil, "a", {c: 1}], Bug::ScanArgs.opt_trail_hash("a", c: 1))
     assert_equal([2, "a", "b", {c: 1}], Bug::ScanArgs.opt_trail_hash("a", "b", c: 1))
-    assert_equal([1, nil, {c: 1}, nil], Bug::ScanArgs.opt_trail_hash(c: 1))
+    assert_warn(/The keyword argument is passed as the last hash parameter/) do
+      assert_equal([1, nil, {c: 1}, nil], Bug::ScanArgs.opt_trail_hash(c: 1))
+    end
     assert_raise(ArgumentError) {Bug::ScanArgs.opt_trail_hash("a", "b", "c")}
-    assert_equal([2, "a", {"b"=>0}, {c: 1}], Bug::ScanArgs.opt_trail_hash("a", "b"=>0, c: 1))
+    assert_warn(/The last argument is split into positional and keyword parameters/) do
+      assert_equal([2, "a", {"b"=>0}, {c: 1}], Bug::ScanArgs.opt_trail_hash("a", "b"=>0, c: 1))
+    end
   end
 
   def test_lead_opt_trail_hash
@@ -173,12 +203,16 @@ class TestScanArgs < Test::Unit::TestCase
     assert_raise(ArgumentError) {Bug::ScanArgs.lead_opt_trail_hash("a")}
     assert_raise(ArgumentError) {Bug::ScanArgs.lead_opt_trail_hash(c: 1)}
     assert_equal([2, "a", nil, "b", nil], Bug::ScanArgs.lead_opt_trail_hash("a", "b"))
-    assert_equal([2, "a", nil, {c: 1}, nil], Bug::ScanArgs.lead_opt_trail_hash("a", c: 1))
+    assert_warn(/The keyword argument is passed as the last hash parameter/) do
+      assert_equal([2, "a", nil, {c: 1}, nil], Bug::ScanArgs.lead_opt_trail_hash("a", c: 1))
+    end
     assert_equal([2, "a", nil, "b", {c: 1}], Bug::ScanArgs.lead_opt_trail_hash("a", "b", c: 1))
     assert_equal([3, "a", "b", "c", nil], Bug::ScanArgs.lead_opt_trail_hash("a", "b", "c"))
     assert_equal([3, "a", "b", "c", {c: 1}], Bug::ScanArgs.lead_opt_trail_hash("a", "b", "c", c: 1))
     assert_raise(ArgumentError) {Bug::ScanArgs.lead_opt_trail_hash("a", "b", "c", "d")}
-    assert_equal([3, "a", "b", {"c"=>0}, {c: 1}], Bug::ScanArgs.lead_opt_trail_hash("a", "b", "c"=>0, c: 1))
+    assert_warn(/The last argument is split into positional and keyword parameters/) do
+      assert_equal([3, "a", "b", {"c"=>0}, {c: 1}], Bug::ScanArgs.lead_opt_trail_hash("a", "b", "c"=>0, c: 1))
+    end
   end
 
   def test_var_trail_hash
@@ -187,45 +221,104 @@ class TestScanArgs < Test::Unit::TestCase
     assert_equal([2, ["a"], "b", nil], Bug::ScanArgs.var_trail_hash("a", "b"))
     assert_equal([1, [], "a", {c: 1}], Bug::ScanArgs.var_trail_hash("a", c: 1))
     assert_equal([2, ["a"], "b", {c: 1}], Bug::ScanArgs.var_trail_hash("a", "b", c: 1))
-    assert_equal([1, [], {c: 1}, nil], Bug::ScanArgs.var_trail_hash(c: 1))
+    assert_warn(/The keyword argument is passed as the last hash parameter/) do
+      assert_equal([1, [], {c: 1}, nil], Bug::ScanArgs.var_trail_hash(c: 1))
+    end
     assert_equal([3, ["a", "b"], "c", nil], Bug::ScanArgs.var_trail_hash("a", "b", "c"))
     assert_equal([3, ["a", "b"], "c", {c: 1}], Bug::ScanArgs.var_trail_hash("a", "b", "c", c: 1))
-    assert_equal([3, ["a", "b"], {"c"=>0}, {c: 1}], Bug::ScanArgs.var_trail_hash("a", "b", "c"=>0, c: 1))
+    assert_warn(/The last argument is split into positional and keyword parameters/) do
+      assert_equal([3, ["a", "b"], {"c"=>0}, {c: 1}], Bug::ScanArgs.var_trail_hash("a", "b", "c"=>0, c: 1))
+    end
   end
 
   def test_lead_var_trail_hash
     assert_raise(ArgumentError) {Bug::ScanArgs.lead_var_trail_hash()}
     assert_raise(ArgumentError) {Bug::ScanArgs.lead_var_trail_hash("a")}
     assert_raise(ArgumentError) {Bug::ScanArgs.lead_var_trail_hash(c: 1)}
-    assert_equal([2, "a", [], {c: 1}, nil], Bug::ScanArgs.lead_var_trail_hash("a", c: 1))
+    assert_warn(/The keyword argument is passed as the last hash parameter/) do
+      assert_equal([2, "a", [], {c: 1}, nil], Bug::ScanArgs.lead_var_trail_hash("a", c: 1))
+    end
     assert_equal([2, "a", [], "b", nil], Bug::ScanArgs.lead_var_trail_hash("a", "b"))
     assert_equal([2, "a", [], "b", {c: 1}], Bug::ScanArgs.lead_var_trail_hash("a", "b", c: 1))
     assert_equal([3, "a", ["b"], "c", nil], Bug::ScanArgs.lead_var_trail_hash("a", "b", "c"))
     assert_equal([3, "a", ["b"], "c", {c: 1}], Bug::ScanArgs.lead_var_trail_hash("a", "b", "c", c: 1))
-    assert_equal([3, "a", ["b"], {"c"=>0}, {c: 1}], Bug::ScanArgs.lead_var_trail_hash("a", "b", c: 1, "c"=>0))
+    assert_warn(/The last argument is split into positional and keyword parameters/) do
+      assert_equal([3, "a", ["b"], {"c"=>0}, {c: 1}], Bug::ScanArgs.lead_var_trail_hash("a", "b", c: 1, "c"=>0))
+    end
   end
 
   def test_opt_var_trail_hash
     assert_raise(ArgumentError) {Bug::ScanArgs.opt_var_trail_hash()}
     assert_equal([1, nil, [], "a", nil], Bug::ScanArgs.opt_var_trail_hash("a"))
-    assert_equal([1, nil, [], {c: 1}, nil], Bug::ScanArgs.opt_var_trail_hash(c: 1))
+    assert_warn(/The keyword argument is passed as the last hash parameter/) do
+      assert_equal([1, nil, [], {c: 1}, nil], Bug::ScanArgs.opt_var_trail_hash(c: 1))
+    end
     assert_equal([1, nil, [], "a", {c: 1}], Bug::ScanArgs.opt_var_trail_hash("a", c: 1))
     assert_equal([2, "a", [], "b", nil], Bug::ScanArgs.opt_var_trail_hash("a", "b"))
     assert_equal([2, "a", [], "b", {c: 1}], Bug::ScanArgs.opt_var_trail_hash("a", "b", c: 1))
     assert_equal([3, "a", ["b"], "c", nil], Bug::ScanArgs.opt_var_trail_hash("a", "b", "c"))
     assert_equal([3, "a", ["b"], "c", {c: 1}], Bug::ScanArgs.opt_var_trail_hash("a", "b", "c", c: 1))
-    assert_equal([3, "a", ["b"], {"c"=>0}, {c: 1}], Bug::ScanArgs.opt_var_trail_hash("a", "b", "c"=>0, c: 1))
+    assert_warn(/The last argument is split into positional and keyword parameters/) do
+      assert_equal([3, "a", ["b"], {"c"=>0}, {c: 1}], Bug::ScanArgs.opt_var_trail_hash("a", "b", "c"=>0, c: 1))
+    end
   end
 
   def test_lead_opt_var_trail_hash
     assert_raise(ArgumentError) {Bug::ScanArgs.lead_opt_var_trail_hash()}
     assert_raise(ArgumentError) {Bug::ScanArgs.lead_opt_var_trail_hash("a")}
-    assert_equal([2, "a", nil, [], {b: 1}, nil], Bug::ScanArgs.lead_opt_var_trail_hash("a", b: 1))
+    assert_warn(/The keyword argument is passed as the last hash parameter/) do
+      assert_equal([2, "a", nil, [], {b: 1}, nil], Bug::ScanArgs.lead_opt_var_trail_hash("a", b: 1))
+    end
     assert_equal([2, "a", nil, [], "b", nil], Bug::ScanArgs.lead_opt_var_trail_hash("a", "b"))
     assert_equal([2, "a", nil, [], "b", {c: 1}], Bug::ScanArgs.lead_opt_var_trail_hash("a", "b", c: 1))
     assert_equal([3, "a", "b", [], "c", nil], Bug::ScanArgs.lead_opt_var_trail_hash("a", "b", "c"))
     assert_equal([3, "a", "b", [], "c", {c: 1}], Bug::ScanArgs.lead_opt_var_trail_hash("a", "b", "c", c: 1))
     assert_equal([4, "a", "b", ["c"], "d", nil], Bug::ScanArgs.lead_opt_var_trail_hash("a", "b", "c", "d"))
-    assert_equal([4, "a", "b", ["c"], {"d"=>0}, {c: 1}], Bug::ScanArgs.lead_opt_var_trail_hash("a", "b", "c", "d"=>0, c: 1))
+    assert_warn(/The last argument is split into positional and keyword parameters/) do
+      assert_equal([4, "a", "b", ["c"], {"d"=>0}, {c: 1}], Bug::ScanArgs.lead_opt_var_trail_hash("a", "b", "c", "d"=>0, c: 1))
+    end
+  end
+
+  def test_k_lead_opt_hash
+    assert_raise(ArgumentError) {Bug::ScanArgs.k_lead_opt_hash}
+    assert_equal([1, "a", nil, {c: 1}], Bug::ScanArgs.k_lead_opt_hash("a", c: 1))
+    assert_equal([1, "a", nil, {c: 1}], Bug::ScanArgs.k_lead_opt_hash("a", {c: 1}))
+    assert_equal([2, "a", "b", {c: 1}], Bug::ScanArgs.k_lead_opt_hash("a", "b", c: 1))
+    assert_equal([2, "a", "b", {c: 1}], Bug::ScanArgs.k_lead_opt_hash("a", "b", {c: 1}))
+    assert_warn(/The keyword argument is passed as the last hash parameter/) do
+      assert_equal([1, {c: 1}, nil, nil], Bug::ScanArgs.k_lead_opt_hash(c: 1))
+    end
+    assert_warn(/The last argument is split into positional and keyword parameters/) do
+      assert_equal([2, "a", {"b"=>0}, {c: 1}], Bug::ScanArgs.k_lead_opt_hash("a", "b"=>0, c: 1))
+    end
+  end
+
+  def test_e_lead_opt_hash
+    assert_warn(/The keyword argument is passed as the last hash parameter/) do
+      assert_equal([1, {}, nil, nil], Bug::ScanArgs.e_lead_opt_hash)
+    end
+    assert_equal([1, "a", nil, nil], Bug::ScanArgs.e_lead_opt_hash("a"))
+    assert_equal([2, "a", "b", nil], Bug::ScanArgs.e_lead_opt_hash("a", "b"))
+    assert_equal([2, "a", {c: 1}, nil], Bug::ScanArgs.e_lead_opt_hash("a", c: 1))
+    assert_raise(ArgumentError) {Bug::ScanArgs.e_lead_opt_hash("a", "b", c: 1)}
+    assert_equal([1, {c: 1}, nil, nil], Bug::ScanArgs.e_lead_opt_hash(c: 1))
+    assert_raise(ArgumentError) {Bug::ScanArgs.e_lead_opt_hash("a", "b", "c")}
+    assert_equal([2, "a", {"b"=>0, c: 1}, nil], Bug::ScanArgs.e_lead_opt_hash("a", "b"=>0, c: 1))
+  end
+
+  def test_n_lead_opt_hash
+    assert_raise(ArgumentError) {Bug::ScanArgs.n_lead_opt_hash}
+    assert_equal([1, "a", nil, nil], Bug::ScanArgs.n_lead_opt_hash("a"))
+    assert_equal([2, "a", "b", nil], Bug::ScanArgs.n_lead_opt_hash("a", "b"))
+    assert_equal([1, "a", nil, {c: 1}], Bug::ScanArgs.n_lead_opt_hash("a", c: 1))
+    assert_equal([1, "a", nil, {c: 1}], Bug::ScanArgs.n_lead_opt_hash("a", {c: 1}))
+    assert_equal([2, "a", "b", {c: 1}], Bug::ScanArgs.n_lead_opt_hash("a", "b", c: 1))
+    assert_equal([2, "a", "b", {c: 1}], Bug::ScanArgs.n_lead_opt_hash("a", "b", {c: 1}))
+    assert_equal([1, {c: 1}, nil, nil], Bug::ScanArgs.n_lead_opt_hash(c: 1))
+    assert_equal([1, {c: 1}, nil, nil], Bug::ScanArgs.n_lead_opt_hash({c: 1}))
+    assert_raise(ArgumentError) {Bug::ScanArgs.n_lead_opt_hash("a", "b", "c")}
+    assert_warn(/The last argument is split into positional and keyword parameters/) do
+      assert_equal([2, "a", {"b"=>0}, {c: 1}], Bug::ScanArgs.n_lead_opt_hash("a", "b"=>0, c: 1))
+    end
   end
 end
diff --git a/test/pathname/test_pathname.rb b/test/pathname/test_pathname.rb
index af523f9f98..053c728247 100644
--- a/test/pathname/test_pathname.rb
+++ b/test/pathname/test_pathname.rb
@@ -933,7 +933,7 @@ class TestPathname < Test::Unit::TestCase
       assert_equal(0444 & ~File.umask, File.stat("b").mode & 0777)
       assert_equal("def", File.read("b"))
 
-      Pathname("c").open("w", 0444, {}) {|f| f.write "ghi" }
+      Pathname("c").open("w", 0444, **{}) {|f| f.write "ghi" }
       assert_equal(0444 & ~File.umask, File.stat("c").mode & 0777)
       assert_equal("ghi", File.read("c"))
 
diff --git a/test/reline/helper.rb b/test/reline/helper.rb
index ca0001258d..b1759f1d80 100644
--- a/test/reline/helper.rb
+++ b/test/reline/helper.rb
@@ -31,7 +31,7 @@ class Reline::TestCase < Test::Unit::TestCase
       if Reline::Unicode::EscapedChars.include?(c.ord)
         c
       else
-        c.encode(@line_editor.instance_variable_get(:@encoding), Encoding::UTF_8, options)
+        c.encode(@line_editor.instance_variable_get(:@encoding), Encoding::UTF_8, **options)
       end
     }.join
   rescue Encoding::UndefinedConversionError, Encoding::InvalidByteSequenceError
diff --git a/test/ruby/test_dir_m17n.rb b/test/ruby/test_dir_m17n.rb
index 7584074c7e..c2c0c4999e 100644
--- a/test/ruby/test_dir_m17n.rb
+++ b/test/ruby/test_dir_m17n.rb
@@ -18,7 +18,7 @@ class TestDir_M17N < Test::Unit::TestCase
         filename = #{code}.chr('UTF-8').force_encoding("#{encoding}")
         File.open(filename, "w") {}
         opts = {:encoding => Encoding.default_external} if /mswin|mingw/ =~ RUBY_PLATFORM
-        ents = Dir.entries(".", opts)
+        ents = Dir.entries(".", **(opts||{}))
         assert_include(ents, filename)
       EOS
 
@@ -26,7 +26,7 @@ class TestDir_M17N < Test::Unit::TestCase
       assert_separately(%w[-EASCII-8BIT], <<-EOS, :chdir=>dir)
         filename = #{code}.chr('UTF-8').force_encoding("ASCII-8BIT")
         opts = {:encoding => Encoding.default_external} if /mswin|mingw/ =~ RUBY_PLATFORM
-        ents = Dir.entries(".", opts)
+        ents = Dir.entries(".", **(opts||{}))
         expected_filename = #{code}.chr('UTF-8').encode(Encoding.find("filesystem")) rescue expected_filename = "?"
         expected_filename = expected_filename.force_encoding("ASCII-8BIT")
         if /mswin|mingw/ =~ RUBY_PLATFORM
@@ -35,7 +35,7 @@ class TestDir_M17N < Test::Unit::TestCase
           when ents.include?(expected_filename)
             filename = expected_filename
           else
-            ents = Dir.entries(".", {:encoding => Encoding.find("filesystem")})
+            ents = Dir.entries(".", :encoding => Encoding.find("filesystem"))
             filename = expected_filename
           end
         end
@@ -52,7 +52,7 @@ class TestDir_M17N < Test::Unit::TestCase
         filename = "\u3042"
         File.open(filename, "w") {}
         opts = {:encoding => Encoding.default_external} if /mswin|mingw/ =~ RUBY_PLATFORM
-        ents = Dir.entries(".", opts)
+        ents = Dir.entries(".", **(opts||{}))
         assert_include(ents, filename)
       EOS
     }
@@ -67,7 +67,7 @@ class TestDir_M17N < Test::Unit::TestCase
         filename = "\xff".force_encoding("ASCII-8BIT") # invalid byte sequence as UTF-8
         File.open(filename, "w") {}
         opts = {:encoding => Encoding.default_external} if /mswin|mingw/ =~ RUBY_PLATFORM
-        ents = Dir.entries(".", opts)
+        ents = Dir.entries(".", **(opts||{}))
         filename = "%FF" if /darwin/ =~ RUBY_PLATFORM && ents.include?("%FF")
         assert_include(ents, filename)
       EOS
@@ -75,7 +75,7 @@ class TestDir_M17N < Test::Unit::TestCase
         filename = "\xff".force_encoding("UTF-8") # invalid byte sequence as UTF-8
         File.open(filename, "w") {}
         opts = {:encoding => Encoding.default_external} if /mswin|mingw/ =~ RUBY_PLATFORM
-        ents = Dir.entries(".", opts)
+        ents = Dir.entries(".", **(opts||{}))
         filename = "%FF" if /darwin/ =~ RUBY_PLATFORM && ents.include?("%FF")
         assert_include(ents, filename)
       EOS
@@ -88,7 +88,7 @@ class TestDir_M17N < Test::Unit::TestCase
         filename = "\xc2\xa1".force_encoding("utf-8")
         File.open(filename, "w") {}
         opts = {:encoding => Encoding.default_external} if /mswin|mingw/ =~ RUBY_PLATFORM
-        ents = Dir.entries(".", opts)
+        ents = Dir.entries(".", **(opts||{}))
         assert_include(ents, filename)
       EOS
       assert_separately(%w[-EUTF-8], <<-'EOS', :chdir=>d)
@@ -125,13 +125,13 @@ class TestDir_M17N < Test::Unit::TestCase
         filename = "\u3042"
         File.open(filename, "w") {}
         opts = {:encoding => Encoding.default_external} if /mswin|mingw/ =~ RUBY_PLATFORM
-        ents = Dir.entries(".", opts)
+        ents = Dir.entries(".", **(opts||{}))
         assert_include(ents, filename)
       EOS
       assert_separately(%w[-EUTF-8:EUC-JP], <<-'EOS', :chdir=>d)
         filename = "\xA4\xA2".force_encoding("euc-jp")
         opts = {:encoding => Encoding.default_external} if /mswin|mingw/ =~ RUBY_PLATFORM
-        ents = Dir.entries(".", opts)
+        ents = Dir.entries(".", **(opts||{}))
         assert_include(ents, filename)
       EOS
       assert_separately(%w[-EUTF-8:EUC-JP], <<-'EOS', :chdir=>d)
@@ -151,7 +151,7 @@ class TestDir_M17N < Test::Unit::TestCase
         File.open(filename1, "w") {}
         File.open(filename2, "w") {}
         opts = {:encoding => Encoding.default_external} if /mswin|mingw/ =~ RUBY_PLATFORM
-        ents = Dir.entries(".", opts)
+        ents = Dir.entries(".", **(opts||{}))
         assert_include(ents, filename1)
         assert_include(ents, filename2)
       EOS
@@ -159,7 +159,7 @@ class TestDir_M17N < Test::Unit::TestCase
         filename1 = "\u2661" # WHITE HEART SUIT which is not representable in EUC-JP
         filename2 = "\xA4\xA2".force_encoding("euc-jp") # HIRAGANA LETTER A in EUC-JP
         opts = {:encoding => Encoding.default_external} if /mswin|mingw/ =~ RUBY_PLATFORM
-        ents = Dir.entries(".", opts)
+        ents = Dir.entries(".", **(opts||{}))
         assert_include(ents, filename1)
         assert_include(ents, filename2)
       EOS
@@ -183,7 +183,7 @@ class TestDir_M17N < Test::Unit::TestCase
         filename = "\xA4\xA2".force_encoding("euc-jp")
         File.open(filename, "w") {}
         opts = {:encoding => Encoding.default_external} if /mswin|mingw/ =~ RUBY_PLATFORM
-        ents = Dir.entries(".", opts)
+        ents = Dir.entries(".", **(opts||{}))
         ents.each {|e| e.force_encoding("ASCII-8BIT") }
         if /darwin/ =~ RUBY_PLATFORM
           filename = filename.encode("utf-8")
@@ -200,7 +200,7 @@ class TestDir_M17N < Test::Unit::TestCase
         filename = "\xA4\xA2".force_encoding("euc-jp")
         File.open(filename, "w") {}
         opts = {:encoding => Encoding.default_external} if /mswin|mingw/ =~ RUBY_PLATFORM
-        ents = Dir.entries(".", opts)
+        ents = Dir.entries(".", **(opts||{}))
         if /darwin/ =~ RUBY_PLATFORM
           filename = filename.encode("utf-8").force_encoding("euc-jp")
         end
@@ -210,14 +210,14 @@ class TestDir_M17N < Test::Unit::TestCase
         filename = "\xA4\xA2".force_encoding('ASCII-8BIT')
         win_expected_filename = filename.encode(Encoding.find("filesystem"), "euc-jp") rescue "?"
         opts = {:encoding => Encoding.default_external} if /mswin|mingw/ =~ RUBY_PLATFORM
-        ents = Dir.entries(".", opts)
+        ents = Dir.entries(".", **(opts||{}))
         unless ents.include?(filename)
           case RUBY_PLATFORM
           when /darwin/
             filename = filename.encode("utf-8", "euc-jp").b
           when /mswin|mingw/
             if ents.include?(win_expected_filename.b)
-              ents = Dir.entries(".", {:encoding => Encoding.find("filesystem")})
+              ents = Dir.entries(".", :encoding => Encoding.find("filesystem"))
               filename = win_expected_filename
             end
           end
@@ -246,7 +246,7 @@ class TestDir_M17N < Test::Unit::TestCase
         filename = "\xA4\xA2".force_encoding("euc-jp")
         File.open(filename, "w") {}
         opts = {:encoding => Encoding.default_external} if /mswin|mingw/ =~ RUBY_PLATFORM
-        ents = Dir.entries(".", opts)
+        ents = Dir.entries(".", **(opts||{}))
         if /darwin/ =~ RUBY_PLATFORM
           filename = filename.encode("utf-8", "euc-jp").force_encoding("euc-jp")
         end
@@ -255,7 +255,7 @@ class TestDir_M17N < Test::Unit::TestCase
       assert_separately(%w[-EEUC-JP:UTF-8], <<-'EOS', :chdir=>d)
         filename = "\u3042"
         opts = {:encoding => Encoding.default_external} if /mswin|mingw/ =~ RUBY_PLATFORM
-        ents = Dir.entries(".", opts)
+        ents = Dir.entries(".", **(opts||{}))
         if /darwin/ =~ RUBY_PLATFORM
           filename = filename.force_encoding("euc-jp")
         end
@@ -420,7 +420,7 @@ class TestDir_M17N < Test::Unit::TestCase
       else
         orig.each {|o| o.force_encoding(enc) }
       end
-      ents = Dir.entries(".", opts).reject {|n| /\A\./ =~ n}
+      ents = Dir.entries(".", **(opts||{})).reject {|n| /\A\./ =~ n}
       ents.sort!
       PP.assert_equal(orig, ents, bug7267)
     }
diff --git a/test/ruby/test_econv.rb b/test/ruby/test_econv.rb
index 6f098db454..115ff73ea8 100644
--- a/test/ruby/test_econv.rb
+++ b/test/ruby/test_econv.rb
@@ -3,7 +3,12 @@ require 'test/unit'
 
 class TestEncodingConverter < Test::Unit::TestCase
   def check_ec(edst, esrc, eres, dst, src, ec, off, len, opts=nil)
-    res = ec.primitive_convert(src, dst, off, len, opts)
+    case opts
+    when Hash
+      res = ec.primitive_convert(src, dst, off, len, **opts)
+    else
+      res = ec.primitive_convert(src, dst, off, len, opts)
+    end
     assert_equal([edst.b, esrc.b, eres],
                  [dst.b,  src.b,  res])
   end
diff --git a/test/ruby/test_exception.rb b/test/ruby/test_exception.rb
index 9de1b2d82c..e5c38091ac 100644
--- a/test/ruby/test_exception.rb
+++ b/test/ruby/test_exception.rb
@@ -1223,6 +1223,14 @@ $stderr = $stdout; raise "\x82\xa0"') do |outs, errs, status|
     assert_equal("#{__FILE__}:#{__LINE__-1}: warning: test warning\n", warning[0])
     assert_raise(ArgumentError) {warn("test warning", uplevel: -1)}
     assert_in_out_err(["-e", "warn 'ok', uplevel: 1"], '', [], /warning:/)
+    warning = capture_warning_warn {warn("test warning", {uplevel: 0})}
+    assert_equal("#{__FILE__}:#{__LINE__-1}: warning: The last argument is used as the keyword parameter\n", warning[0])
+    assert_match(/warning: for method defined here|warning: test warning/, warning[1])
+    warning = capture_warning_warn {warn("test warning", **{uplevel: 0})}
+    assert_equal("#{__FILE__}:#{__LINE__-1}: warning: test warning\n", warning[0])
+    warning = capture_warning_warn {warn("test warning", {uplevel: 0}, **{})}
+    assert_equal("test warning\n{:uplevel=>0}\n", warning[0])
+    assert_raise(ArgumentError) {warn("test warning", foo: 1)}
   end
 
   def test_warning_warn_invalid_argument
diff --git a/test/ruby/test_io.rb b/test/ruby/test_io.rb
index ed60452b36..a610b608aa 100644
--- a/test/ruby/test_io.rb
+++ b/test/ruby/test_io.rb
@@ -423,7 +423,7 @@ class TestIO < Test::Unit::TestCase
     path = t.path
     t.close!
     assert_raise(Errno::ENOENT, "[ruby-dev:33072]") do
-      File.read(path, nil, nil, {})
+      File.read(path, nil, nil, **{})
     end
   end
 
@@ -2489,15 +2489,15 @@ class TestIO < Test::Unit::TestCase
       assert_equal(["foo\n", "bar\n", "baz\n"], a)
 
       a = []
-      IO.foreach(t.path, {:mode => "r" }) {|x| a << x }
+      IO.foreach(t.path, :mode => "r") {|x| a << x }
       assert_equal(["foo\n", "bar\n", "baz\n"], a)
 
       a = []
-      IO.foreach(t.path, {:open_args => [] }) {|x| a << x }
+      IO.foreach(t.path, :open_args => []) {|x| a << x }
       assert_equal(["foo\n", "bar\n", "baz\n"], a)
 
       a = []
-      IO.foreach(t.path, {:open_args => ["r"] }) {|x| a << x }
+      IO.foreach(t.path, :open_args => ["r"]) {|x| a << x }
       assert_equal(["foo\n", "bar\n", "baz\n"], a)
 
       a = []
@@ -3119,7 +3119,9 @@ __END__
       assert_equal(1, File.write(path, "f", 0, encoding: "UTF-8"))
       assert_equal("ff", File.read(path))
       assert_raise(TypeError) {
-        File.write(path, "foo", Object.new => Object.new)
+        assert_warn(/The last argument is split into positional and keyword parameters/) do
+          File.write(path, "foo", Object.new => Object.new)
+        end
       }
     end
   end
diff --git a/test/ruby/test_io_m17n.rb b/test/ruby/test_io_m17n.rb
index 8101bfb62f..022ff330b5 100644
--- a/test/ruby/test_io_m17n.rb
+++ b/test/ruby/test_io_m17n.rb
@@ -23,7 +23,8 @@ class TestIO_M17N < Test::Unit::TestCase
 
   def pipe(*args, wp, rp)
     re, we = nil, nil
-    r, w = IO.pipe(*args)
+    kw = args.last.is_a?(Hash) ? args.pop : {}
+    r, w = IO.pipe(*args, **kw)
     rt = Thread.new do
       begin
         rp.call(r)
diff --git a/test/ruby/test_literal.rb b/test/ruby/test_literal.rb
index 3f87f860b1..7f4a329c4a 100644
--- a/test/ruby/test_literal.rb
+++ b/test/ruby/test_literal.rb
@@ -189,7 +189,7 @@ class TestRubyLiteral < Test::Unit::TestCase
     def test_debug_frozen_string
       src = 'n = 1; _="foo#{n ? "-#{n}" : ""}"'; f = "test.rb"; n = 1
       opt = {frozen_string_literal: true, debug_frozen_string_literal: true}
-      str = RubyVM::InstructionSequence.compile(src, f, f, n, opt).eval
+      str = RubyVM::InstructionSequence.compile(src, f, f, n, **opt).eval
       assert_equal("foo-1", str)
       assert_predicate(str, :frozen?)
       assert_raise_with_message(FrozenError, /created at #{Regexp.quote(f)}:#{n}/) {
@@ -200,7 +200,7 @@ class TestRubyLiteral < Test::Unit::TestCase
     def test_debug_frozen_string_in_array_literal
       src = '["foo"]'; f = "test.rb"; n = 1
       opt = {frozen_string_literal: true, debug_frozen_string_literal: true}
-      ary = RubyVM::InstructionSequence.compile(src, f, f, n, opt).eval
+      ary = RubyVM::InstructionSequence.compile(src, f, f, n, **opt).eval
       assert_equal("foo", ary.first)
       assert_predicate(ary.first, :frozen?)
       assert_raise_with_message(FrozenError, /created at #{Regexp.quote(f)}:#{n}/) {
diff --git a/test/ruby/test_numeric.rb b/test/ruby/test_numeric.rb
index 6073ec1ee5..f87abbb4a8 100644
--- a/test/ruby/test_numeric.rb
+++ b/test/ruby/test_numeric.rb
@@ -230,7 +230,8 @@ class TestNumeric < Test::Unit::TestCase
   end
 
   def assert_step(expected, (from, *args), inf: false)
-    enum = from.step(*args)
+    kw = args.last.is_a?(Hash) ? args.pop : {}
+    enum = from.step(*args, **kw)
     size = enum.size
     xsize = expected.size
 
@@ -239,7 +240,7 @@ class TestNumeric < Test::Unit::TestCase
       assert_send [size, :>, 0], "step size: +infinity"
 
       a = []
-      from.step(*args) { |x| a << x; break if a.size == xsize }
+      from.step(*args, **kw) { |x| a << x; break if a.size == xsize }
       assert_equal expected, a, "step"
 
       a = []
@@ -249,7 +250,7 @@ class TestNumeric < Test::Unit::TestCase
       assert_equal expected.size, size, "step size"
 
       a = []
-      from.step(*args) { |x| a << x }
+      from.step(*args, **kw) { |x| a << x }
       assert_equal expected, a, "step"
 
       a = []
diff --git a/test/ruby/test_string.rb b/test/ruby/test_string.rb
index 36d246f9e0..dd403bf9d9 100644
--- a/test/ruby/test_string.rb
+++ b/test/ruby/test_string.rb
@@ -17,8 +17,8 @@ class TestString < Test::Unit::TestCase
     super
   end
 
-  def S(*args)
-    @cls.new(*args)
+  def S(*args, **kw)
+    @cls.new(*args, **kw)
   end
 
   def test_s_new