c++: Fix ICE on nonsense requires-clause.

Here we were swallowing all the syntax errors by parsing tentatively, and
returning error_mark_node without ever actually giving an error.  Fixed by
using save_tokens/rollback_tokens instead.

	PR c++/92517
	* parser.c (cp_parser_constraint_primary_expression): Do the main
	parse non-tentatively.

gcc/cp/ChangeLog

+2020-02-06  Jason Merrill  <jason@redhat.com>
+
+	PR c++/92517
+	* parser.c (cp_parser_constraint_primary_expression): Do the main
+	parse non-tentatively.
+
 2020-02-06  Marek Polacek  <polacek@redhat.com>
 
 	PR c++/93597 - ICE with lambda in operator function.

gcc/cp/parser.c

@@ -27478,7 +27478,7 @@ cp_parser_constraint_primary_expression (cp_parser *parser, bool lambda_p)
       return e;
     }
 
-  cp_parser_parse_tentatively (parser);
+  cp_lexer_save_tokens (parser->lexer);
   cp_id_kind idk;
   location_t loc = input_location;
   cp_expr expr = cp_parser_primary_expression (parser,
@@ -27494,19 +27494,16 @@ cp_parser_constraint_primary_expression (cp_parser *parser, bool lambda_p)
       /* The primary-expression could be part of an unenclosed non-logical
 	 compound expression.  */
       pce = cp_parser_constraint_requires_parens (parser, lambda_p);
-      if (pce != pce_ok)
-	cp_parser_simulate_error (parser);
-      else
-	expr = finish_constraint_primary_expr (expr);
     }
-  if (cp_parser_parse_definitely (parser))
-    return expr;
-  if (expr == error_mark_node)
-    return error_mark_node;
+  if (pce == pce_ok)
+    {
+      cp_lexer_commit_tokens (parser->lexer);
+      return finish_constraint_primary_expr (expr);
+    }
 
   /* Retry the parse at a lower precedence. If that succeeds, diagnose the
      error, but return the expression as if it were valid.  */
-  gcc_assert (pce != pce_ok);
+  cp_lexer_rollback_tokens (parser->lexer);
   cp_parser_parse_tentatively (parser);
   if (pce == pce_maybe_operator)
     expr = cp_parser_assignment_expression (parser, NULL, false, false);

gcc/testsuite/g++.dg/cpp2a/concepts-syntax1.C

+// PR c++/92517
+// { dg-do compile { target concepts } }
+
+template <typename T>
+concept C = true;
+
+template<int I>
+requires C decltype<I>		// { dg-error "" }
+void f() {}