Simplify the fix for bug #41530

We don’t need to upgrade the target string and redo the pattern match
if the replacement is in utf8.  We can simply convert during concate-
nation, using the more recently added SV_CATUTF8 and SV_CATBYTES flags
to sv_catpvn_flags.

This should make things faster, too, as sv_catpvn_flags does not need
to allocate extra SVs or string buffers.

This happened to trigger an existing COW bug, causing test failures.
SvIsCOW and sv_force_normal_flags were being called on TARG before
get-magic.  So a magical scalar returning a COW could have that COW
modified in place.

I added a test for something I nearly broke.

1 files changed, 2 insertions, 0 deletions

diff --git a/sv.h b/sv.h
index 69a73801b0..4cac64a6ea 100644
--- a/sv.h
+++ b/sv.h
@@ -1819,6 +1819,8 @@ mg.c:1024: warning: left-hand operand of comma expression has no effect
 	    sv_utf8_upgrade(nsv);			\
 	    sv_catsv_nomg(dsv, nsv);			\
 	} STMT_END
+#define sv_catpvn_nomg_maybeutf8(dsv, sstr, slen, is_utf8) \
+	sv_catpvn_flags(dsv, sstr, slen, (is_utf8)?SV_CATUTF8:SV_CATBYTES)
 
 #ifdef PERL_CORE
 # define sv_or_pv_len_utf8(sv, pv, bytelen)	      \