pad.c: Fix GH Issue #19463, -DXv fails assert when dumping anonymous …

…constant sub

Anonymous constant subs were changed to be implemented internally as
XSUBs in 5.21.6, commit 1567c65.
This broke DEBUGGING perls running under -DXv which weren't taught
about the new implementation. In ed958fa
strict.pm also was changed to use such subs, which then breaks many
uses of -DXv.

See t/run/switchDx.t for an example of code that would trigger this that
does not depend on strict.pm

This fixes the problem and adds a test for -Dx.

pad.c

@@ -1858,7 +1858,6 @@ STATIC void
 S_cv_dump(pTHX_ const CV *cv, const char *title)
 {
     const CV * const outside = CvOUTSIDE(cv);
-    PADLIST* const padlist = CvPADLIST(cv);
 
     PERL_ARGS_ASSERT_CV_DUMP;
 
@@ -1878,9 +1877,15 @@ S_cv_dump(pTHX_ const CV *cv, const char *title)
                    : CvUNIQUE(outside) ? "UNIQUE"
                    : CvGV(outside) ? GvNAME(CvGV(outside)) : "UNDEFINED"));
 
-    PerlIO_printf(Perl_debug_log,
+    if (!CvISXSUB(cv)) {
+        /* SVPADLIST(cv) will fail an assert if CvISXSUB(cv) is true,
+         * and if the assert is removed this code will SEGV. XSUBs don't
+         * have padlists I believe - Yves */
+        PADLIST* const padlist = CvPADLIST(cv);
+        PerlIO_printf(Perl_debug_log,
                     "    PADLIST = 0x%" UVxf "\n", PTR2UV(padlist));
-    do_dump_pad(1, Perl_debug_log, padlist, 1);
+        do_dump_pad(1, Perl_debug_log, padlist, 1);
+    }
 }
 
 #endif /* DEBUGGING */

t/run/switchDx.t

@@ -14,7 +14,7 @@ skip_all "DEBUGGING build required"
   unless $::Config{ccflags} =~ /(?<!\S)-DDEBUGGING(?!\S)/
          or $^O eq 'VMS' && $::Config{usedebugging_perl} eq 'Y';
 
-plan tests => 8;
+plan tests => 9;
 
 END {
     unlink $perlio_log;
@@ -47,4 +47,10 @@ END {
                     { stderr => 1, switches => [ '-TDi' ] },
                    "Perlio debug output to STDERR with -TDi (no PERLIO_DEBUG)");
 }
-
+{
+    # -DXv tests
+    fresh_perl_like('{ my $n=1; *foo= sub () { $n }; }',
+                    qr/To: CV=0x[a-f0-9]+ \(ANON\), OUTSIDE=0x0 \(null\)/,
+                    { stderr => 1, switches => [ '-DXv' ] },
+                    "-DXv does not assert when dumping anonymous constant sub");
+}