stop lc() etc accidentally modifying in-place.

As an optimisation, [ul]c() and [ul]cfirst() sometimes modify their
argument in-place rather than returning a modified copy.

This should only be done when there is no possibility that the arg is
going to be reused. However, this fails:

    use List::Util qw{ first };
    my %hash = ( ASD => 1, ZXC => 2, QWE => 3, TYU => 4);
    print first { lc $_ eq 'qwe' } keys %hash;

which prints "qwe" rather than "QWE".

Bascally everything in perl that sets $_ or $a/$b and calls a code block
or function, such as map, grep, for and, sort, either copies any PADTMPs,
turns off SvTEMP, and/or bumps the reference count.

List::Util doesn't do this, and it is likely that other CPAN modules
which do "set $_ and call a block" don't either.

This has been failing since 5.20.0: perl has been in-placing if the arg is
(SvTEMP && RC==1 && !mg) (due to v5.19.7-112-g5cd5e2d).

Make the optimisation critera stricter by always copying SvTEMPs.
It still allows the optimisation if the arg is a PADTMP - I don't know
whether this is unsafe too.

Perhaps we can think of something better after 5.24?

1 files changed, 3 insertions, 11 deletions

diff --git a/pp.c b/pp.c
index 4fcc57736f..4a2cde05e0 100644
--- a/pp.c
+++ b/pp.c
@@ -3875,10 +3875,7 @@ PP(pp_ucfirst)
     /* We may be able to get away with changing only the first character, in
      * place, but not if read-only, etc.  Later we may discover more reasons to
      * not convert in-place. */
-    inplace = !SvREADONLY(source)
-	   && (  SvPADTMP(source)
-	      || (  SvTEMP(source) && !SvSMAGICAL(source)
-		 && SvREFCNT(source) == 1));
+    inplace = !SvREADONLY(source) && SvPADTMP(source);
 
     /* First calculate what the changed first character should be.  This affects
      * whether we can just swap it out, leaving the rest of the string unchanged,
@@ -4118,9 +4115,7 @@ PP(pp_uc)
 
     SvGETMAGIC(source);
 
-    if ((SvPADTMP(source)
-	 ||
-	(SvTEMP(source) && !SvSMAGICAL(source) && SvREFCNT(source) == 1))
+    if (   SvPADTMP(source)
 	&& !SvREADONLY(source) && SvPOK(source)
 	&& !DO_UTF8(source)
 	&& (
@@ -4377,10 +4372,7 @@ PP(pp_lc)
 
     SvGETMAGIC(source);
 
-    if (   (  SvPADTMP(source)
-	   || (  SvTEMP(source) && !SvSMAGICAL(source)
-	      && SvREFCNT(source) == 1  )
-	   )
+    if (   SvPADTMP(source)
 	&& !SvREADONLY(source) && SvPOK(source)
 	&& !DO_UTF8(source)) {