hash.c: Do not use the fast path (rb_yield_values) for lambda blocks

As a semantics, Hash#each yields a 2-element array (pairs of keys and
values).  So, `{ a: 1 }.each(&->(k, v) { })` should raise an exception
due to lambda's arity check.
However, the optimization that avoids Array allocation by using
rb_yield_values for blocks whose arity is more than 1 (introduced at
b9d29603375d17c3d1d609d9662f50beaec61fa1 and some commits), seemed to
overlook the lambda case, and wrongly allowed the code above to work.

This change experimentally attempts to make it strict; now the code
above raises an ArgumentError.  This is an incompatible change; if the
compatibility issue is bigger than our expectation, it may be reverted
(until Ruby 3.0 release).

[Bug #12706]

1 files changed, 1 insertions, 1 deletions

diff --git a/struct.c b/struct.c
index 79131db2bd..3042d3bed5 100644
--- a/struct.c
+++ b/struct.c
@@ -821,7 +821,7 @@ rb_struct_each_pair(VALUE s)
 
     RETURN_SIZED_ENUMERATOR(s, 0, 0, struct_enum_size);
     members = rb_struct_members(s);
-    if (rb_block_arity() > 1) {
+    if (rb_block_pair_yield_optimizable()) {
 	for (i=0; i<RSTRUCT_LEN(s); i++) {
 	    VALUE key = rb_ary_entry(members, i);
 	    VALUE value = RSTRUCT_GET(s, i);