PR c++/62314: add fixit hint for missing "template <> " in explicit specialization

gcc/cp/ChangeLog:
	PR c++/62314
	* parser.c (cp_parser_class_head): Capture the start location;
	use it to emit a fix-it insertion hint when complaining
	about missing "template <> " in explicit specializations.

gcc/testsuite/ChangeLog:
	PR c++/62314
	* g++.dg/pr62314.C: New test case.

From-SVN: r235777

gcc/cp/ChangeLog

+2016-05-02  David Malcolm  <dmalcolm@redhat.com>
+
+	PR c++/62314
+	* parser.c (cp_parser_class_head): Capture the start location;
+	use it to emit a fix-it insertion hint when complaining
+	about missing "template <> " in explicit specializations.
+
 2016-05-02  Richard Sandiford  <richard.sandiford@arm.com>
 
 	* init.c (build_new_1): Use shift operators instead of wi:: shifts.

gcc/cp/parser.c

@@ -21656,6 +21656,8 @@ cp_parser_class_head (cp_parser* parser,
   if (class_key == none_type)
     return error_mark_node;
 
+  location_t class_head_start_location = input_location;
+
   /* Parse the attributes.  */
   attributes = cp_parser_attributes_opt (parser);
 
@@ -21872,8 +21874,20 @@ cp_parser_class_head (cp_parser* parser,
       && parser->num_template_parameter_lists == 0
       && template_id_p)
     {
-      error_at (type_start_token->location,
-		"an explicit specialization must be preceded by %<template <>%>");
+      /* Build a location of this form:
+           struct typename <ARGS>
+           ^~~~~~~~~~~~~~~~~~~~~~
+         with caret==start at the start token, and
+         finishing at the end of the type.  */
+      location_t reported_loc
+        = make_location (class_head_start_location,
+                         class_head_start_location,
+                         get_finish (type_start_token->location));
+      rich_location richloc (line_table, reported_loc);
+      richloc.add_fixit_insert (class_head_start_location, "template <> ");
+      error_at_rich_loc
+        (&richloc,
+         "an explicit specialization must be preceded by %<template <>%>");
       invalid_explicit_specialization_p = true;
       /* Take the same action that would have been taken by
 	 cp_parser_explicit_specialization.  */

gcc/testsuite/ChangeLog

+2016-05-02  David Malcolm  <dmalcolm@redhat.com>
+
+	PR c++/62314
+	* g++.dg/pr62314.C: New test case.
+
 2016-05-02  Jan Hubicka  <hubicka@ucw.cz>
 
 	* gcc.dg/ipa/inline-8.c: New testcase.

gcc/testsuite/g++.dg/pr62314.C

+// { dg-options "-fdiagnostics-show-caret" }
+
+template <typename T>
+struct iterator_traits {};
+
+struct file_iterator;
+
+struct iterator_traits<file_iterator> { // { dg-error "explicit specialization must be preceded by .template" }
+};
+
+/* Verify that we emit a fixit hint for this case.  */
+
+/* { dg-begin-multiline-output "" }
+ struct iterator_traits<file_iterator>
+ ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+ template <> 
+   { dg-end-multiline-output "" } */