Fix assertion failure with qr/\Q(?{})/

\Q and \u  create ops that need targets, and hence use the pad of
the anonymous sub created temporarily when parsing something like
qr/\Q(?{})/.  If it turns out we don’t have a code block (in this
case), that anon sub is thrown away, but there is an assertion that
makes sure its pad has not been used, which fails:

$ ./perl -e 'qr/\Q(?{})/'
Assertion failed: (AvFILLp(PL_comppad) == 0), function Perl_pmruntime, file op.c, line 5395.
Abort trap: 6

(That assertion was added by d63c20f27.)

If we have had \Q or \l, then the length of the pad may be more than
1, but constant folding should have stolen the values from the pad, so
assert that instead.

2 files changed, 16 insertions, 3 deletions

diff --git a/op.c b/op.c
index fdf2a0311a..a3391a27ed 100644
--- a/op.c
+++ b/op.c
@@ -5391,8 +5391,17 @@ Perl_pmruntime(pTHX_ OP *o, OP *expr, bool isreg, I32 floor)
 		 * were wrong (e.g. /[(?{}]/ ). Throw away the PL_compcv
 		 * that isn't required now. Note that we have to be pretty
 		 * confident that nothing used that CV's pad while the
-		 * regex was parsed */
-		assert(AvFILLp(PL_comppad) == 0); /* just @_ */
+		 * regex was parsed, except maybe op targets for \Q etc.
+		 * If there were any op targets, though, they should have
+		 * been stolen by constant folding.
+		 */
+#ifdef DEBUGGING
+		PADOFFSET i = 0;
+		assert(PadnamelistMAXNAMED(PL_comppad_name) == 0);
+		while (++i <= AvFILLp(PL_comppad)) {
+		    assert(!PL_curpad[i]);
+		}
+#endif
 		/* But we know that one op is using this CV's slab. */
 		cv_forget_slab(PL_compcv);
 		LEAVE_SCOPE(floor);
diff --git a/t/re/rxcode.t b/t/re/rxcode.t
index aa2ea092d8..d043f447d5 100644
--- a/t/re/rxcode.t
+++ b/t/re/rxcode.t
@@ -6,7 +6,7 @@ BEGIN {
     set_up_inc('../lib');
 }
 
-plan tests => 41;
+plan tests => 42;
 
 $^R = undef;
 like( 'a',  qr/^a(?{1})(?:b(?{2}))?/, 'a =~ ab?' );
@@ -98,3 +98,7 @@ cmp_ok( scalar(@var), '==', 0, '..still nothing pushed (package)' );
 
 @a = 1..3;
 like eval { qr/@a(?{})/ }, qr/1 2 3\(\?\{\}\)/, 'qr/@a(?{})/';
+
+# Not a code block, but looks a bit like one.  (Failed an assertion from
+# 5.17.1 to 5.21.6.)
+ok "(?{" =~ qr/\Q(?{/, 'qr/\Q(?{/';