[perl #118693] Remove PADTMP exemption from uninit warnings

This fixes a problem with undefined values return from XSUBs not pro-
ducing such warnings.

The default typemap for XSUBs uses the target of the entersub call (in
the caller’s pad) to return the converted value, instead of having to
allocate a new SV for that.

So, for example, a function returning char* will cause that char* to
be assigned to the target via sv_setpv.  Then the target is returned.

As a special case, NULL return from a char*-returning function will
produce an undef return value.  This undef return value was not trig-
gering an uninitialized warning.

All targets are marked PADTMP, and anything marked PADTMP is exempt
from uninitialized warnings in some code paths, but not others.

This goes all the way back to 91bba347, which suppressed the warning
with only a hit at why (something to do with bitwise ops warning inap-
propriately).  I think it was to make ~undef exempt.  But a1afd104
stopped it from being exempt.

Only a few pieces of code were relying on this exemption, and it was
hiding bugs, too.  The last few commits have addressed those, so kiss
this exemption good-bye!

pp_reverse had a workaround to force an uninit warning (since
1e21d011c), so remove the workaround to avoid a double uninit warning.

3 files changed, 16 insertions, 2 deletions

diff --git a/ext/XS-Typemap/Typemap.pm b/ext/XS-Typemap/Typemap.pm
index 7b83e1d04a..de1cc6dff5 100644
--- a/ext/XS-Typemap/Typemap.pm
+++ b/ext/XS-Typemap/Typemap.pm
@@ -66,7 +66,7 @@ $VERSION = '0.10';
            T_FLOAT
            T_NV
 	   T_DOUBLE
-	   T_PV
+	   T_PV T_PV_null
 	   T_PTR_IN T_PTR_OUT
 	   T_PTRREF_IN T_PTRREF_OUT
 	   T_REF_IV_REF
diff --git a/ext/XS-Typemap/Typemap.xs b/ext/XS-Typemap/Typemap.xs
index dd34c39bd6..a43c843838 100644
--- a/ext/XS-Typemap/Typemap.xs
+++ b/ext/XS-Typemap/Typemap.xs
@@ -571,6 +571,13 @@ T_PV( in )
  OUTPUT:
   RETVAL
 
+char *
+T_PV_null()
+ CODE:
+  RETVAL = NULL;
+ OUTPUT:
+  RETVAL
+
 
 ## T_PTR
 
diff --git a/ext/XS-Typemap/t/Typemap.t b/ext/XS-Typemap/t/Typemap.t
index 0717801150..ef22280b70 100644
--- a/ext/XS-Typemap/t/Typemap.t
+++ b/ext/XS-Typemap/t/Typemap.t
@@ -6,7 +6,7 @@ BEGIN {
     }
 }
 
-use Test::More tests => 146;
+use Test::More tests => 148;
 
 use strict;
 use warnings;
@@ -211,6 +211,13 @@ is( sprintf("%6.3f",T_DOUBLE(52.345)), sprintf("%6.3f",52.345), "T_DOUBLE" );
 note("T_PV");
 is( T_PV("a string"), "a string");
 is( T_PV(52), 52);
+ok !defined T_PV_null, 'RETVAL = NULL returns undef for char*';
+{
+    my $uninit;
+    local $SIG{__WARN__} = sub { ++$uninit if shift =~ /uninit/ };
+    () = ''.T_PV_null;
+    is $uninit, 1, 'uninit warning from NULL returned from char* func';
+}
 
 # T_PTR
 my $t = 5;