Reset method caches when GPs are shared

The new MRO stuff in 5.10 made PL_sub_generation++ mostly unnecessary,
and almost all uses of it were replaced with mro_method_changed_in.

There is only one problem: That doesn’t actually work properly.  After
glob-to-glob assignment (*foo = *bar), both globs share the same GP
(glob pointer, or list of glob slots).  But there is no list of GVs
associated with any GP.  So there is no way, given a GV whose GP
is shared, to find out what other classes might need their method
caches reset.

sub B::b { "b" }
*A::b = *B::b;
@C::ISA = "A";
print C->b, "\n";  # should print "b"
eval 'sub B::b { "c" }';
print C->b, "\n";  # should print "c"
__END__

$ perl5.8.9 foo
b
c
$ perl5.10.0 foo
b
b

And it continues up to 5.16.x.

If a GP is shared, then those places where mro_method_changed_in is
called after the GP has been modified must do PL_sub_generation++
instead if the GP is shared, which can be detected through its refer-
ence count.

1 files changed, 1 insertions, 1 deletions

diff --git a/sv.c b/sv.c
index a2d0cbc204..8570efb401 100644
--- a/sv.c
+++ b/sv.c
@@ -3823,7 +3823,7 @@ S_glob_assign_ref(pTHX_ SV *const dstr, SV *const sstr)
 	    }
 	    GvCVGEN(dstr) = 0; /* Switch off cacheness. */
 	    GvASSUMECV_on(dstr);
-	    if(GvSTASH(dstr)) mro_method_changed_in(GvSTASH(dstr)); /* sub foo { 1 } sub bar { 2 } *bar = \&foo */
+	    if(GvSTASH(dstr)) gv_method_changed(dstr); /* sub foo { 1 } sub bar { 2 } *bar = \&foo */
 	}
 	*location = SvREFCNT_inc_simple_NN(sref);
 	if (import_flag && !(GvFLAGS(dstr) & import_flag)