[perl #24237] @& should not stop $& from working

Mentioning $& in a program slows everything down, because it force
regular expressions to do a pre-match copy.

It used to happen for any symbol named &, e.g., @& and %&.  This was
changed in commit b4a9608f339, but that commit did not take into
account that the code path in question is only followed on creation of
the *& glob.

It should still be applying magic to $&, even if it is not setting
PL_sawampersand.  The other place in gv_fetchpvn_flags that magical-
ises scalars (which currently handles %- %+ %! $] and @ISA), should
also turn on PL_sawampersand for $&.

All of the above applies to $' and $` as well.

2 files changed, 18 insertions, 5 deletions

diff --git a/gv.c b/gv.c
index df3735a105..4b4d9a6529 100644
--- a/gv.c
+++ b/gv.c
@@ -1650,8 +1650,14 @@ Perl_gv_fetchpvn_flags(pTHX_ const char *nambeg, STRLEN full_len, I32 flags,
 		else if (*name == '-' || *name == '+')
 		    require_tie_mod(gv, name, newSVpvs("Tie::Hash::NamedCapture"), "TIEHASH", 0);
 	      }
-	      if ((sv_type==SVt_PV || sv_type==SVt_PVGV) && *name == '[')
+	      if (sv_type==SVt_PV || sv_type==SVt_PVGV) {
+	       if (*name == '[')
 		require_tie_mod(gv,name,newSVpvs("arybase"),"FETCH",0);
+	       else if (*name == '&' || *name == '`' || *name == '\'') {
+		PL_sawampersand = TRUE;
+		(void)GvSVn(gv);
+	       }
+	      }
 	    }
 	    else if (len == 3 && sv_type == SVt_PVAV
 	          && strnEQ(name, "ISA", 3)
@@ -1859,14 +1865,13 @@ Perl_gv_fetchpvn_flags(pTHX_ const char *nambeg, STRLEN full_len, I32 flags,
 	case '&':		/* $& */
 	case '`':		/* $` */
 	case '\'':		/* $' */
-	    if (
+	    if (!(
 		sv_type == SVt_PVAV ||
 		sv_type == SVt_PVHV ||
 		sv_type == SVt_PVCV ||
 		sv_type == SVt_PVFM ||
 		sv_type == SVt_PVIO
-		) { break; }
-	    PL_sawampersand = TRUE;
+		)) { PL_sawampersand = TRUE; }
 	    goto magicalize;
 
 	case ':':		/* $: */
diff --git a/t/op/magic.t b/t/op/magic.t
index adbdad60d9..6ddf2d8071 100644
--- a/t/op/magic.t
+++ b/t/op/magic.t
@@ -5,7 +5,7 @@ BEGIN {
     chdir 't' if -d 't';
     @INC = '../lib';
     require './test.pl';
-    plan (tests => 150);
+    plan (tests => 153);
 }
 
 # Test that defined() returns true for magic variables created on the fly,
@@ -180,6 +180,14 @@ is $&, 'bar';
 is $', 'baz';
 is $+, 'a';
 
+# [perl #24237]
+for (qw < ` & ' >) {
+ fresh_perl_is
+  qq < \@$_; q "fff" =~ /(?!^)./; print "[\$$_]\\n" >,
+  "[f]\n", {},
+  "referencing \@$_ before \$$_ etc. still saws off ampersands";
+}
+
 # $"
 @a = qw(foo bar baz);
 is "@a", "foo bar baz";