c++: GCC accepts junk before fold-expression [PR86773]

Here we accept a bogus expression before a left fold:

Recall that a fold expression looks like:

 fold-expression:
    ( cast-expression fold-operator ... )
    ( ... fold-operator cast-expression )
    ( cast-expression fold-operator ... fold-operator cast-expression )

but here we have

    ( cast-expression ... fold-operator cast-expression )

The best fix seems to just return error_mark_node when we know this code
is invalid, and let the subsequent code report that a ) was expected.

gcc/cp/ChangeLog:

	PR c++/86773
	* parser.c (cp_parser_fold_expression): Return error_mark_node
	if a left fold is preceded by an expression.

gcc/testsuite/ChangeLog:

	PR c++/86773
	* g++.dg/cpp1z/fold12.C: New test.

gcc/cp/parser.c

@@ -5138,6 +5138,8 @@ cp_parser_fold_expression (cp_parser *parser, tree expr1)
   // Left fold.
   if (cp_lexer_next_token_is (parser->lexer, CPP_ELLIPSIS))
     {
+      if (expr1)
+	return error_mark_node;
       cp_lexer_consume_token (parser->lexer);
       int op = cp_parser_fold_operator (parser);
       if (op == ERROR_MARK)

gcc/testsuite/g++.dg/cpp1z/fold12.C

@@ -0,0 +1,13 @@
+// PR c++/86773
+// { dg-do compile { target c++17 } }
+
+template <typename ... Param>
+auto work(Param && ...param)
+{
+  return ("asda" ... / param); // { dg-error "expected" }
+}
+
+int main()
+{
+  work(1.0, 2.0, 5, 4.0);
+}