Handle empty subroutine signatures in parse_subsignature() (Fixes #17689)

Author: leonerd

ext/XS-APItest/t/subsignature.t

@@ -16,6 +16,12 @@ eval q{
 	push @t, (subsignature @rest);
 	push @t, (subsignature %rest);
 	push @t, (subsignature $one = 1);
+
+	# these should all appear empty
+	push @t, (subsignature );
+	push @t, (subsignature);
+	push @t, (subsignature #empty
+		    );
 };
 is $@, "";
 is_deeply \@t, [
@@ -24,6 +30,10 @@ is_deeply \@t, [
 	['nextstate:6', 'multiparam:0..0:@:@rest=*'],
 	['nextstate:7', 'multiparam:0..0:%:%rest=*'],
 	['nextstate:8', 'multiparam:0..1:-:$one=0?'],
+
+	['nextstate:11', 'multiparam:0..0:-'],
+	['nextstate:12', 'multiparam:0..0:-'],
+	['nextstate:13', 'multiparam:0..0:-'],
 ];
 
 done_testing;

pod/perldelta.pod

@@ -412,6 +412,16 @@ manager will later use a regex to expand these into links.
 
 XXX
 
+=item * C<parse_subsignature()> can now handle empty subroutine signatures
+
+Previously, calling the C<parse_subsignature()> API function with an empty
+signature would cause a "Syntax error" failure, requiring code which calls it
+to detect the special case and take appropriate steps.  This is now fixed,
+returning the same optree that the parser would yield for a regular empty
+signature during normal parse time.
+
+[GH#17689]
+
 =back
 
 =head1 Known Problems

toke.c

@@ -13922,6 +13922,18 @@ Perl_parse_subsignature(pTHX_ U32 flags)
 {
     if (flags)
         croak("Parsing code internal error (%s)", "parse_subsignature");
+    /* sub signatures might be empty, but perly.y can't cope with the
+     * ambiguity of an empty construction. We'll detect it manually and do
+     * something suitable in that case. */
+    lex_read_space(0);
+    if (lex_peek_unichar(0) == ')') {
+        /* pretend we saw an empty signature and do the same steps perly.y
+         * would have performed. */
+        subsignature_start();
+        OP *sigops = subsignature_finish();
+        CvSIGNATURE_on(PL_compcv);
+        return sigops;
+    }
     return parse_recdescent_for_op(GRAMSUBSIGNATURE, LEX_FAKEEOF_NONEXPR);
 }