openmp: Diagnose omp::directive attribute without balanced token argument [PR102413]

If omp::directive attribute argument starting with the opening ( is not a balanced
token sequence, then cp_parser_skip_balanced_tokens (parser, 1) returns 1,
but the code was subtracting 2 from it and iterating until it was 0, so for the
non-balanced case it iterated from (size_t) -1 down to 0.

The following patch just diagnoses that as an error.

2021-09-23  Jakub Jelinek  <jakub@redhat.com>

	PR c++/102413
	* parser.c (cp_parser_omp_directive_args): Diagnose if omp::directive
	is not followed by a balanced token sequence starting with open paren.

	* g++.dg/gomp/attrs-14.C: New test.

gcc/cp/parser.c

@@ -28628,7 +28628,16 @@ cp_parser_omp_directive_args (cp_parser *parser, tree attribute)
       TREE_VALUE (attribute) = NULL_TREE;
       return;
     }
-  for (size_t n = cp_parser_skip_balanced_tokens (parser, 1) - 2; n; --n)
+  size_t n = cp_parser_skip_balanced_tokens (parser, 1);
+  if (n == 1)
+    {
+      cp_lexer_consume_token (parser->lexer);
+      error_at (first->location, "expected attribute argument as balanced "
+				 "token sequence");
+      TREE_VALUE (attribute) = NULL_TREE;
+      return;
+    }
+  for (n = n - 2; n; --n)
     cp_lexer_consume_token (parser->lexer);
   cp_token *last = cp_lexer_peek_token (parser->lexer);
   cp_lexer_consume_token (parser->lexer);

gcc/testsuite/g++.dg/gomp/attrs-14.C

@@ -0,0 +1,4 @@
+// PR c++/102413
+// { dg-do compile { target c++11 } }
+
+[[omp::directive(error]];	// { dg-error "expected|declare" }