c++: Fix attributes with lambda and trailing return type.

My fix for 60503 fixed handling of C++11 attributes following the
lambda-declarator.  My patch for 89640 re-added support for GNU attributes,
but attributes after the trailing return type were parsed as applying to the
return type rather than to the function.  This patch adjusts parsing of a
trailing-return-type to ignore GNU attributes at the end of the declaration
so that they will be applied to the declaration as a whole.

I also considered parsing the attributes between the closing paren and the
trailing-return-type, and tried a variety of approaches to implementing
that, but I think it's better to stick with the documented rule that "An
attribute specifier list may appear immediately before the comma, '=' or
semicolon terminating the declaration of an identifier...."  Anyone
disagree?

Meanwhile, C++ committee discussion about the lack of any way to apply
attributes to a lambda op() seems to have concluded that they should go
between the introducer and declarator, so I've implemented that as well.

	PR c++/90333
	PR c++/89640
	PR c++/60503
	* parser.c (cp_parser_type_specifier_seq): Don't parse attributes in
	a trailing return type.
	(cp_parser_lambda_declarator_opt): Parse C++11 attributes before
	parens.

gcc/cp/ChangeLog

+2020-01-29  Jason Merrill  <jason@redhat.com>
+
+	PR c++/90333
+	PR c++/89640
+	PR c++/60503
+	* parser.c (cp_parser_type_specifier_seq): Don't parse attributes in
+	a trailing return type.
+	(cp_parser_lambda_declarator_opt): Parse C++11 attributes before
+	parens.
+
 2020-01-29  Marek Polacek  <polacek@redhat.com>
 
 	PR c++/91754 - Fix template arguments comparison with class NTTP.

gcc/cp/parser.c

@@ -10962,6 +10962,9 @@ cp_parser_lambda_declarator_opt (cp_parser* parser, tree lambda_expr)
       ++parser->num_template_parameter_lists;
     }
 
+  /* Committee discussion supports allowing attributes here.  */
+  lambda_specs.attributes = cp_parser_attributes_opt (parser);
+
   /* The parameter-declaration-clause is optional (unless
      template-parameter-list was given), but must begin with an
      opening parenthesis if present.  */
@@ -11097,7 +11100,7 @@ cp_parser_lambda_declarator_opt (cp_parser* parser, tree lambda_expr)
 
     fco = grokmethod (&return_type_specs,
 		      declarator,
-		      gnu_attrs);
+		      chainon (gnu_attrs, lambda_specs.attributes));
     if (fco != error_mark_node)
       {
 	DECL_INITIALIZED_IN_CLASS_P (fco) = 1;
@@ -22277,6 +22280,18 @@ cp_parser_type_specifier_seq (cp_parser* parser,
       /* Check for attributes first.  */
       if (cp_next_tokens_can_be_attribute_p (parser))
 	{
+	  /* GNU attributes at the end of a declaration apply to the
+	     declaration as a whole, not to the trailing return type.  So look
+	     ahead to see if these attributes are at the end.  */
+	  if (seen_type_specifier && is_trailing_return
+	      && cp_next_tokens_can_be_gnu_attribute_p (parser))
+	    {
+	      size_t n = cp_parser_skip_attributes_opt (parser, 1);
+	      cp_token *tok = cp_lexer_peek_nth_token (parser->lexer, n);
+	      if (tok->type == CPP_SEMICOLON || tok->type == CPP_COMMA
+		  || tok->type == CPP_EQ || tok->type == CPP_OPEN_BRACE)
+		break;
+	    }
 	  type_specifier_seq->attributes
 	    = attr_chainon (type_specifier_seq->attributes,
 			    cp_parser_attributes_opt (parser));

gcc/testsuite/g++.dg/ext/attr-trailing1.C

+// PR c++/90333
+// { dg-do compile { target c++11 } }
+
+auto l = [] [[nodiscard]] () -> int { return 0; };
+auto l2 = []() -> int __attribute ((warn_unused_result)) { return 0; };
+auto f() -> int __attribute ((warn_unused_result));
+auto f() -> int { return 0; }
+
+int main()
+{
+  l();				// { dg-warning "nodiscard" }
+  l2();				// { dg-warning "unused_result" }
+  f();				// { dg-warning "unused_result" }
+}