Flatten vstrings modified in place

A substitution forces its target to a string upon successful substitu-
tion, even if the substitution did nothing:

$ ./perl -Ilib -le '$a = *f; $a =~ s/f/f/; print ref \$a'
SCALAR

Notice that $a is no longer a glob after s///.

But vstrings are different:

$ ./perl -Ilib -le '$a = v102; $a =~ s/f/f/; print ref \$a'
VSTRING

I fixed this in 5.16 (1e6bda93) for those cases where the vstring ends
up with a value that doesn’t correspond to the actual string:

$ ./perl -Ilib -le '$a = v102; $a =~ s/f/o/; print ref \$a'
SCALAR

It works through vstring set-magic, that does the check and removes
the magic if it doesn’t match.

I did it that way because I couldn’t think of any other way to fix
bug #29070, and I didn’t realise at the time that I hadn’t fixed
all the bugs.

By making SvTHINKFIRST true on a vstring, we force it through
sv_force_normal before any in-place string operations.  We can also
make sv_force_normal handle vstrings as well.  This fixes all the lin-
gering-vstring-magic bugs in just two lines, making the vstring set-
magic (which is also slow) redundant.  It also allows the special case
in sv_setsv_flags to be removed.

Or at least that was what I had hoped.

It turns out that pp_subst, twists and turns in tortuous ways, and
needs special treatment for things like this.

And do_trans functions wasn’t checking SvTHINKFIRST when arguably it
should have.

I tweaked sv_2pv{utf8,byte} to avoid copying magic variables that do
not need copying.

1 files changed, 1 insertions, 1 deletions

diff --git a/mg_raw.h b/mg_raw.h
index 76cf42f13d..f577087434 100644
--- a/mg_raw.h
+++ b/mg_raw.h
@@ -68,7 +68,7 @@
       "/* taint 't' Taintedness */" },
     { 'U', "want_vtbl_uvar",
       "/* uvar 'U' Available for use by extensions */" },
-    { 'V', "want_vtbl_vstring | PERL_MAGIC_VALUE_MAGIC",
+    { 'V', "magic_vtable_max | PERL_MAGIC_VALUE_MAGIC",
       "/* vstring 'V' SV was vstring literal */" },
     { 'v', "want_vtbl_vec | PERL_MAGIC_VALUE_MAGIC",
       "/* vec 'v' vec() lvalue */" },