c++: Fix ICE-on-invalid with broken attribute [PR93684]

We crash when parsing

  [[a::

because we see a CPP_SCOPE and then we're trying to consume a CPP_EOF
token.  So peek before consuming it.

	PR c++/93684 - ICE-on-invalid with broken attribute.
	* parser.c (cp_parser_std_attribute): Peek a token first before
	consuming it.

	* g++.dg/parse/attr4.C: New test.

gcc/cp/ChangeLog

+2020-02-12  Marek Polacek  <polacek@redhat.com>
+
+	PR c++/93684 - ICE-on-invalid with broken attribute.
+	* parser.c (cp_parser_std_attribute): Peek a token first before
+	consuming it.
+
 2020-02-11  Jason Merrill  <jason@redhat.com>
 
 	PR c++/93675

gcc/cp/parser.c

@@ -26720,7 +26720,7 @@ cp_parser_std_attribute (cp_parser *parser, tree attr_ns)
 				   "with scoped attribute token");
       attr_ns = attr_id;
 
-      token = cp_lexer_consume_token (parser->lexer);
+      token = cp_lexer_peek_token (parser->lexer);
       if (token->type == CPP_NAME)
 	attr_id = token->u.value;
       else if (token->type == CPP_KEYWORD)
@@ -26733,6 +26733,7 @@ cp_parser_std_attribute (cp_parser *parser, tree attr_ns)
 		    "expected an identifier for the attribute name");
 	  return error_mark_node;
 	}
+      cp_lexer_consume_token (parser->lexer);
 
       attr_ns = canonicalize_attr_name (attr_ns);
       attr_id = canonicalize_attr_name (attr_id);

gcc/testsuite/ChangeLog

+2020-02-12  Marek Polacek  <polacek@redhat.com>
+
+	PR c++/93684 - ICE-on-invalid with broken attribute.
+	* g++.dg/parse/attr4.C: New test.
+
 2020-02-12  Jakub Jelinek  <jakub@redhat.com>
 
 	PR target/93670

gcc/testsuite/g++.dg/parse/attr4.C

+// PR c++/93684 - ICE-on-invalid with broken attribute.
+
+[[a:: // { dg-error "expected|expected" }