[gcc(refs/users/marxin/heads/marxin-gcc-benchmark-branch)] c++: Diagnose when "requires" is used instead of "requires requires" [PR94306]

[Martin Liska]

https://gcc.gnu.org/g:7981c06ae92548bd66f07121db1802eb6aec73ed

commit 7981c06ae92548bd66f07121db1802eb6aec73ed
Author: Patrick Palka <ppalka@redhat.com>
Date:   Sat Mar 28 08:57:11 2020 -0400

    c++: Diagnose when "requires" is used instead of "requires requires" [PR94306]
    
    This adds support to detect and recover from the case where an opening brace
    immediately follows the start of a requires-clause.  So rather than emitting the
    error
    
      error: expected primary-expression before '{' token
    
    followed by a slew of irrevelant errors, we now assume the user had intended to
    write "requires requires {" and diagnose and recover accordingly.
    
    gcc/cp/ChangeLog:
    
            PR c++/94306
            * parser.c (cp_parser_requires_clause_opt): Diagnose and recover from
            "requires {" when "requires requires {" was probably intended.
    
    gcc/testsuite/ChangeLog:
    
            PR c++/94306
            * g++.dg/concepts/diagnostic8.C: New test.

Diff:
---
 gcc/cp/ChangeLog                            |  4 ++++
 gcc/cp/parser.c                             | 17 ++++++++++++++++-
 gcc/testsuite/ChangeLog                     |  3 +++
 gcc/testsuite/g++.dg/concepts/diagnostic8.C |  6 ++++++
 4 files changed, 29 insertions(+), 1 deletion(-)

diff --git a/gcc/cp/ChangeLog b/gcc/cp/ChangeLog
index a197c7d14e2..e7c248a3c2f 100644
--- a/gcc/cp/ChangeLog
+++ b/gcc/cp/ChangeLog
@@ -1,5 +1,9 @@
 2020-03-28  Patrick Palka  <ppalka@redhat.com>
 
+	PR c++/94306
+	* parser.c (cp_parser_requires_clause_opt): Diagnose and recover from
+	"requires {" when "requires requires {" was probably intended.
+
 	PR c++/94252
 	* constraint.cc (tsubst_compound_requirement): Always suppress errors
 	from type_deducible_p and expression_convertible_p, as they're not
diff --git a/gcc/cp/parser.c b/gcc/cp/parser.c
index bf7387dd149..fc3c2976e3d 100644
--- a/gcc/cp/parser.c
+++ b/gcc/cp/parser.c
@@ -27704,7 +27704,22 @@ cp_parser_requires_clause_opt (cp_parser *parser, bool lambda_p)
 	}
       return NULL_TREE;
     }
-  cp_lexer_consume_token (parser->lexer);
+
+  cp_token *tok2 = cp_lexer_peek_nth_token (parser->lexer, 2);
+  if (tok2->type == CPP_OPEN_BRACE)
+    {
+      /* An opening brace following the start of a requires-clause is
+	 ill-formed; the user likely forgot the second `requires' that
+	 would start a requires-expression.  */
+      gcc_rich_location richloc (tok2->location);
+      richloc.add_fixit_insert_after (tok->location, " requires");
+      error_at (&richloc, "missing additional %<requires%> to start "
+		"a requires-expression");
+      /* Don't consume the `requires', so that it's reused as the start of a
+	 requires-expression.  */
+    }
+  else
+    cp_lexer_consume_token (parser->lexer);
 
   if (!flag_concepts_ts)
     return cp_parser_requires_clause_expression (parser, lambda_p);
diff --git a/gcc/testsuite/ChangeLog b/gcc/testsuite/ChangeLog
index 8c8eee014fd..3422550ae02 100644
--- a/gcc/testsuite/ChangeLog
+++ b/gcc/testsuite/ChangeLog
@@ -1,5 +1,8 @@
 2020-03-28  Patrick Palka  <ppalka@redhat.com>
 
+	PR c++/94306
+	* g++.dg/concepts/diagnostic8.C: New test.
+
 	PR c++/94252
 	* g++.dg/concepts/diagnostic7.C: New test.
 	* g++.dg/concepts/pr94252.C: New test.
diff --git a/gcc/testsuite/g++.dg/concepts/diagnostic8.C b/gcc/testsuite/g++.dg/concepts/diagnostic8.C
new file mode 100644
index 00000000000..70d7e4a9cc1
--- /dev/null
+++ b/gcc/testsuite/g++.dg/concepts/diagnostic8.C
@@ -0,0 +1,6 @@
+// PR c++/94306
+// { dg-do compile { target c++2a } }
+
+template<typename T> struct S { };
+template<typename T> requires { typename T::type; } struct S<T> { };
+// { dg-error "missing additional .requires." "" { target *-*-* } .-1 }