[perl #119055] Make qq with no vars read-only

In commit 2484f8db I stopped constant folding from changing the read-
onliness of expressions like 1+2, which should return mutable values,
seeing that ${\1}+2 returns a mutable value.  (After all, constant
folding is supposed to be solely an optimisation, without changing
behaviour.)

This is accomplished by turning on the PADTMP flag, which tells opera-
tors like \ to copy the scalar.

I did not realise at the time that some qq strings like "hello\n" and
qq "foo" (but not just "foo") are implemented using constant folding.
The lexer outputs something like stringify("foo"), which is compiled
down to a stringify op with a constant ("foo") as its only child.

In this case we really do want qq"foo" to be treated as a single con-
stant.  That it is implemented using folding while "foo" is not is an
implementation detail we should hide.

So turn off the PADTMP flag when folding a stringify op.

2 files changed, 10 insertions, 2 deletions

diff --git a/op.c b/op.c
index 9ae081213f..55bfbf0e1e 100644
--- a/op.c
+++ b/op.c
@@ -3333,7 +3333,8 @@ S_fold_constants(pTHX_ OP *o)
     op_free(o);
 #endif
     assert(sv);
-    if (!SvIMMORTAL(sv)) SvPADTMP_on(sv);
+    if (type == OP_STRINGIFY) SvPADTMP_off(sv);
+    else if (!SvIMMORTAL(sv)) SvPADTMP_on(sv);
     if (type == OP_RV2GV)
 	newop = newGVOP(OP_GV, 0, MUTABLE_GV(sv));
     else
diff --git a/t/comp/fold.t b/t/comp/fold.t
index c483c99c93..64d4d2356c 100644
--- a/t/comp/fold.t
+++ b/t/comp/fold.t
@@ -4,7 +4,7 @@
 # we've not yet verified that use works.
 # use strict;
 
-print "1..27\n";
+print "1..28\n";
 my $test = 0;
 
 # Historically constant folding was performed by evaluating the ops, and if
@@ -158,3 +158,10 @@ for(1+2) {
           " - 1+2 returns mutable value, just like \$a+\$b",
           "\n";
 }
+
+# [perl #119055]
+# We hide the implementation detail that qq "foo" is implemented using
+# constant folding.
+eval { ${\"hello\n"}++ };
+print "not " unless $@ =~ "Modification of a read-only value attempted at";
+print "ok ", ++$test, " - qq with no vars is a constant\n";