Revert "[perl #79908] Stop sub inlining from breaking closures"

This reverts commit 137da2b05b4b7628115049f343163bdaf2c30dbb. See the
"How about having a recommended way to add constant subs dynamically?"
thread on perl5-porters, specifically while it sucks that we have this
bug, it's been documented to work this way since 5.003 in "Constant
Functions" in perlsub:

    If the result after optimization and constant folding is either a
    constant or a lexically-scoped scalar which has no other references,
    then it will be used in place of function calls made without C<&>

    -- http://perldoc.perl.org/perlsub.html#Constant-Functions

Since we've had this documented bug for a long time we should introduce
this fix in a deprecation cycle rather than silently slowing down code
that assumes it's going to be optimized by constant folding.

I didn't revert the tests it t/op/sub.t, but turned them into TODO tests
instead.

Conflicts:
	t/op/sub.t

1 files changed, 19 insertions, 0 deletions

diff --git a/pad.c b/pad.c
index 419b40338d..31282d157d 100644
--- a/pad.c
+++ b/pad.c
@@ -2197,6 +2197,25 @@ S_cv_clone(pTHX_ CV *proto, CV *cv, CV *outside)
 	cv_dump(cv,	 "To");
     );
 
+    if (CvCONST(cv)) {
+	/* Constant sub () { $x } closing over $x - see lib/constant.pm:
+	 * The prototype was marked as a candiate for const-ization,
+	 * so try to grab the current const value, and if successful,
+	 * turn into a const sub:
+	 */
+	SV* const const_sv = op_const_sv(CvSTART(cv), cv);
+	if (const_sv) {
+	    SvREFCNT_dec_NN(cv);
+            /* For this calling case, op_const_sv returns a *copy*, which we
+               donate to newCONSTSUB. Yes, this is ugly, and should be killed.
+               Need to fix how lib/constant.pm works to eliminate this.  */
+	    cv = newCONSTSUB(CvSTASH(proto), NULL, const_sv);
+	}
+	else {
+	    CvCONST_off(cv);
+	}
+    }
+
     return cv;
 }