Make empty lvalue subs work correctly

In perl 5.8.1 and earlier, sub{} would return @_ in list context. This
was fixed in 5.8.2 for regular subs, but not lvalue subs.

Before the syntactic restriction on return values was removed in
commit 145b2bb, there was a bug affecting compilation of empty subs
before any use statement:

$ perl5.14.0 -e 'sub foo :lvalue {}'
Can't modify stub in lvalue subroutine return at -e line 1, near "{}"
Execution of -e aborted due to compilation errors.
$ perl5.14.0 -le 'use sigtrap; sub foo :lvalue {} print "ok"'
ok

But I digress. :-)

Up to 5.14, lvalue subs were still returning @_, or, rather, the ele-
ments of @_ as separate scalars:

$ perl5.14.0 -Mre -le '(sub :lvalue {}->($a,$b))=(3,4); print "$a $b"'
Useless use of "re" pragma at -e line 0
3 4

(Not exactly useless, eh? The -Mre allows the sub to compile.)

This commit fixes that bug.

1 files changed, 11 insertions, 3 deletions

diff --git a/pp_hot.c b/pp_hot.c
index 2525a26d39..bb0f20517b 100644
--- a/pp_hot.c
+++ b/pp_hot.c
@@ -2745,14 +2745,22 @@ PP(pp_leavesublv)
 		    SvREFCNT_inc_void(*mark);
 		}
 	    }
-	    else {			/* Should not happen? */
+	    else {
+		/* sub:lvalue{} will take us here.
+		   Presumably the case of a non-empty array never happens.
+		 */
 		LEAVE;
 		cxstack_ix--;
 		POPSUB(cx,sv);
 		PL_curpm = newpm;
 		LEAVESUB(sv);
-		DIE(aTHX_ "%s returned from lvalue subroutine in scalar context",
-		    (MARK > SP ? "Empty array" : "Array"));
+		DIE(aTHX_ "%s",
+		    (MARK > SP
+		      ? "Can't return undef from lvalue subroutine"
+		      : "Array returned from lvalue subroutine in scalar "
+		        "context"
+		    )
+		);
 	    }
 	    SP = MARK;
 	}