Warn when refalias desynchronises a closure-captured variable

The `refalias` feature is currently experimental due to a number of
limitations of its implementation, most notably the effect it has on
closures such as the one illustrated in this unit test. Any change in
implementation to make this case function as intended would be a large
undertaking, so instead as a path towards being able to remove the
experimental status from this feature, we consider this behaviour
warning instead.

Author: leonerd

pod/perldiag.pod

@@ -8328,6 +8328,15 @@ anonymous, using the C<sub {}> syntax.  When inner anonymous subs that
 reference variables in outer subroutines are created, they
 are automatically rebound to the current values of such variables.
 
+=item Variable "%s" will not stay shared after refalias
+
+(W closure) A refalias operation has been performed on a variable that is
+captured by one or more closures, and thus is shared between them.  As a
+result of the way it is currently implemented, the variable in the scope
+that performed has now become disconnected from those in other scopes, and
+now operates independently.  Updates to this variable will not be reflected
+in the others, or vice versa.
+
 =item vector argument not supported with alpha versions
 
 (S printf) The %vd (s)printf format does not support version objects

pp.c

@@ -7624,6 +7624,12 @@ PP(pp_refassign)
     case 0:
     {
         SV * const old = PAD_SV(ARGTARG);
+        if(SvREFCNT(old) > 1) {
+            PADNAME *pn = PadlistNAMESARRAY(CvPADLIST(find_runcv(NULL)))[ARGTARG];
+            warner(packWARN(WARN_CLOSURE),
+                "Variable \"%" PNf "\" will not stay shared after refalias",
+                PNfARG(pn));
+        }
         PAD_SETSV(ARGTARG, SvREFCNT_inc_NN(SvRV(sv)));
         SvREFCNT_dec(old);
         if ((PL_op->op_private & (OPpLVAL_INTRO|OPpPAD_STATE))

t/op/lvref.t

@@ -5,7 +5,7 @@ BEGIN {
     set_up_inc("../lib");
 }
 
-plan 203;
+plan 204;
 
 eval '\$x = \$y';
 like $@, qr/^Experimental aliasing via reference not enabled/,
@@ -626,6 +626,9 @@ like $@,
 # Miscellaneous
 
 {
+  my $warnings = "";
+  local $SIG{__WARN__} = sub { $warnings .= $_[0]; };
+
   local $::TODO = ' ';
   my($x,$y);
   sub {
@@ -635,6 +638,10 @@ like $@,
     is \$x, \$y, 'lexical alias affects outer closure';
   }->();
   is \$x, \$y, 'lexical alias affects outer sub where vars are declared';
+
+  undef $::TODO;
+  like $warnings, qr/^Variable "\$x" will not stay shared after refalias /,
+    'warning emitted by refalias on closure shared variable';
 }
 
 { # PADSTALE has a double meaning