IO#set_encoding_by_bom should err when encoding is already set

Except for ASCII-8BIT.  [Bug #16422]

1 files changed, 4 insertions, 0 deletions

diff --git a/io.c b/io.c
index 63d8172e07..eaaccbbe07 100644
--- a/io.c
+++ b/io.c
@@ -8334,6 +8334,10 @@ rb_io_set_encoding_by_bom(VALUE io)
     if (fptr->encs.enc2) {
         rb_raise(rb_eArgError, "encoding conversion is set");
     }
+    else if (fptr->encs.enc && fptr->encs.enc != rb_ascii8bit_encoding()) {
+        rb_raise(rb_eArgError, "encoding is set to %s already",
+                 rb_enc_name(fptr->encs.enc));
+    }
     if (!io_set_encoding_by_bom(io)) return Qnil;
     return rb_enc_from_encoding(fptr->encs.enc);
 }