[pushed] c++: ICE on invalid concept placeholder [PR94481].

[Jason Merrill]

Here the 'decltype' is missing '(auto)', so open_paren was NULL, and trying
to get its location is a SEGV.  Using matching_parens avoids that problem.

Tested x86_64-pc-linux-gnu, applying to trunk.

gcc/cp/ChangeLog
2020-04-07  Jason Merrill  <jason@redhat.com>

	PR c++/94481
	* parser.c (cp_parser_placeholder_type_specifier): Use
	matching_parens.
---
 gcc/cp/parser.c                                    | 12 +++++-------
 gcc/testsuite/g++.dg/cpp2a/concepts-placeholder2.C |  9 +++++++++
 2 files changed, 14 insertions(+), 7 deletions(-)
 create mode 100644 gcc/testsuite/g++.dg/cpp2a/concepts-placeholder2.C

diff --git a/gcc/cp/parser.c b/gcc/cp/parser.c
index fbcdc9bb5fc..2c33ca4dd16 100644
--- a/gcc/cp/parser.c
+++ b/gcc/cp/parser.c
@@ -18367,7 +18367,7 @@ cp_parser_placeholder_type_specifier (cp_parser *parser, location_t loc,
 
   /* As per the standard, require auto or decltype(auto), except in some
      cases (template parameter lists, -fconcepts-ts enabled).  */
-  cp_token *placeholder = NULL, *open_paren = NULL, *close_paren = NULL;
+  cp_token *placeholder = NULL, *close_paren = NULL;
   if (cxx_dialect >= cxx2a)
     {
       if (cp_lexer_next_token_is_keyword (parser->lexer, RID_AUTO))
@@ -18375,12 +18375,10 @@ cp_parser_placeholder_type_specifier (cp_parser *parser, location_t loc,
       else if (cp_lexer_next_token_is_keyword (parser->lexer, RID_DECLTYPE))
 	{
 	  placeholder = cp_lexer_consume_token (parser->lexer);
-	  open_paren = cp_parser_require (parser, CPP_OPEN_PAREN,
-					  RT_OPEN_PAREN);
+	  matching_parens parens;
+	  parens.require_open (parser);
 	  cp_parser_require_keyword (parser, RID_AUTO, RT_AUTO);
-          close_paren = cp_parser_require (parser, CPP_CLOSE_PAREN,
-					   RT_CLOSE_PAREN,
-					   open_paren->location);
+	  close_paren = parens.require_close (parser);
 	}
     }
 
@@ -18429,7 +18427,7 @@ cp_parser_placeholder_type_specifier (cp_parser *parser, location_t loc,
      results in an invented template parameter.  */
   if (parser->auto_is_implicit_function_template_parm_p)
     {
-      if (placeholder && token_is_decltype (placeholder))
+      if (close_paren)
 	{
 	  location_t loc = make_location (placeholder->location,
 					  placeholder->location,
diff --git a/gcc/testsuite/g++.dg/cpp2a/concepts-placeholder2.C b/gcc/testsuite/g++.dg/cpp2a/concepts-placeholder2.C
new file mode 100644
index 00000000000..b04354cf8d0
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp2a/concepts-placeholder2.C
@@ -0,0 +1,9 @@
+// PR c++/94481
+// { dg-do compile { target c++2a } }
+
+template<typename T>
+concept C = true;
+
+void foo() {
+  C decltype c = 1;		// { dg-error "" }
+}

base-commit: c23c899aedf11069e992eed7358802b262d62f98
-- 
2.18.1