Fix qx, `` and <<`` overrides

This resolves two RT tickets:
• #115330 is that qx and `` overrides do not support interpolation.
• #119827 is that <<`` does not support readpipe overrides at all.

The obvious fix for #115330 fixes #119827 at the same time.

When quote-like operators are parsed, after the string has been
scanned S_sublex_push is called, which decides which of two paths
to follow:
1) For things not requiring interpolation, the string is passed to
   tokeq (originally called q, it handles double backslashes and back-
   slashed delimiters) and returned to the parser immediately.
2) For anything that interpolates, the lexer enters a special inter-
   polation mode (LEX_INTERPPUSH) and goes through a more complex
   sequence over the next few calls (e.g., qq"a.$b.c" is turned into
  ‘stringify ( "a." . $ b . ".c" )’).

When commit e3f73d4ed (Oct 2006, perl 5.10) added support for overrid-
ing `` and qx with a readpipe sub, it did so by creating an entersub
op in toke.c and making S_sublex_push follow path no. 1, taking the
result if tokeq and inserting it into the already-constructed op tree
for the sub call.

That approach caused interpolation to be skipped when qx or `` is
overridden.  Furthermore it didn’t touch <<`` at all.

The easiest solution is to let toke.c follow its normal path and
create a backtick op (instead of trying to half-intercept it), and
to deal with override lookup afterwards in ck_backtick, the same way
require overrides are handled.  Since <<`` also turns into a backtick
op, it gets handled too that way.

1 files changed, 12 insertions, 1 deletions

diff --git a/t/op/exec.t b/t/op/exec.t
index 28f3043094..6ec3646c4d 100644
--- a/t/op/exec.t
+++ b/t/op/exec.t
@@ -36,7 +36,7 @@ $ENV{LANGUAGE} = 'C';		# Ditto in GNU.
 my $Is_VMS   = $^O eq 'VMS';
 my $Is_Win32 = $^O eq 'MSWin32';
 
-plan(tests => 22);
+plan(tests => 24);
 
 my $Perl = which_perl();
 
@@ -129,6 +129,17 @@ END
     is( readpipe, "ok\n", 'readpipe default argument' );
 }
 
+package o {
+    use subs "readpipe";
+    sub readpipe { pop }
+    ::is `${\"hello"}`, 'hello',
+         'overridden `` interpolates [perl #115330]';
+    ::is <<`119827`, "ls\n",
+l${\"s"}
+119827
+        '<<`` respects overrides and interpolates [perl #119827]';
+}
+
 TODO: {
     my $tnum = curr_test();
     if( $^O =~ /Win32/ ) {