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.

gcc/cp/ChangeLog

 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

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);

gcc/testsuite/ChangeLog

 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.

gcc/testsuite/g++.dg/concepts/diagnostic8.C

+// 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 }