c++: Allow attributes on concepts - DR 2428

The following patch adds parsing of attributes to concept definition,
allows deprecated attribute to be specified (as CONCEPT_DECL now needs
to be checked in c-family/c-attribs.cc, I had to move its declaration
from cp/*.def to c-family/*.def) and checks TREE_DEPRECATED in
build_standard_check (not sure if that is the right spot, or whether
it shouldn't be checked also for variable and function concepts and
how to write testcase coverage for that).

2022-11-16  Jakub Jelinek  <jakub@redhat.com>

gcc/c-family/
	* c-common.def (CONCEPT_DECL): New tree, moved here from
	cp-tree.def.
	* c-common.cc (c_common_init_ts): Handle CONCEPT_DECL.
	* c-attribs.cc (handle_deprecated_attribute): Allow deprecated
	attribute on CONCEPT_DECL.
gcc/cp/
	* cp-tree.def (CONCEPT_DECL): Move to c-common.def.
	* cp-objcp-common.cc (cp_common_init_ts): Don't handle CONCEPT_DECL
	here.
	* cp-tree.h (finish_concept_definition): Add ATTRS parameter.
	* parser.cc (cp_parser_concept_definition): Parse attributes in
	between identifier and =.  Adjust finish_concept_definition
	caller.
	* pt.cc (finish_concept_definition): Add ATTRS parameter.  Call
	cplus_decl_attributes.
	* constraint.cc (build_standard_check): If CONCEPT_DECL is
	TREE_DEPRECATED, emit -Wdeprecated-declaration warnings.
gcc/testsuite/
	* g++.dg/cpp2a/concepts-dr2428.C: New test.

6 files changed, 10 insertions, 9 deletions

diff --git a/gcc/cp/constraint.cc b/gcc/cp/constraint.cc
index 3ddbd53..a113d3e 100644
--- a/gcc/cp/constraint.cc
+++ b/gcc/cp/constraint.cc
@@ -1396,6 +1396,8 @@ build_standard_check (tree tmpl, tree args, tsubst_flags_t complain)
 {
   gcc_assert (standard_concept_p (tmpl));
   gcc_assert (TREE_CODE (tmpl) == TEMPLATE_DECL);
+  if (TREE_DEPRECATED (DECL_TEMPLATE_RESULT (tmpl)))
+    warn_deprecated_use (DECL_TEMPLATE_RESULT (tmpl), NULL_TREE);
   tree parms = INNERMOST_TEMPLATE_PARMS (DECL_TEMPLATE_PARMS (tmpl));
   args = coerce_template_parms (parms, args, tmpl, complain);
   if (args == error_mark_node)
diff --git a/gcc/cp/cp-objcp-common.cc b/gcc/cp/cp-objcp-common.cc
index e4df30d..7f76f2c 100644
--- a/gcc/cp/cp-objcp-common.cc
+++ b/gcc/cp/cp-objcp-common.cc
@@ -473,7 +473,6 @@ cp_common_init_ts (void)
   /* New decls.  */
   MARK_TS_DECL_COMMON (TEMPLATE_DECL);
   MARK_TS_DECL_COMMON (WILDCARD_DECL);
-  MARK_TS_DECL_COMMON (CONCEPT_DECL);
 
   MARK_TS_DECL_NON_COMMON (USING_DECL);
 
diff --git a/gcc/cp/cp-tree.def b/gcc/cp/cp-tree.def
index f83b4c5..3de8278 100644
--- a/gcc/cp/cp-tree.def
+++ b/gcc/cp/cp-tree.def
@@ -495,11 +495,6 @@ DEFTREECODE (OMP_DEPOBJ, "omp_depobj", tcc_statement, 2)
 
 /* Extensions for Concepts. */
 
-/* Concept definition. This is not entirely different than a VAR_DECL
-   except that a) it must be a template, and b) doesn't have the wide
-   range of value and linkage options available to variables.  */
-DEFTREECODE (CONCEPT_DECL, "concept_decl", tcc_declaration, 0)
-
 /* Used to represent information associated with constrained declarations. */
 DEFTREECODE (CONSTRAINT_INFO, "constraint_info", tcc_exceptional, 0)
 
diff --git a/gcc/cp/cp-tree.h b/gcc/cp/cp-tree.h
index 8c9beb8..07f96ea 100644
--- a/gcc/cp/cp-tree.h
+++ b/gcc/cp/cp-tree.h
@@ -8322,7 +8322,7 @@ struct diagnosing_failed_constraint
 extern cp_expr finish_constraint_or_expr	(location_t, cp_expr, cp_expr);
 extern cp_expr finish_constraint_and_expr	(location_t, cp_expr, cp_expr);
 extern cp_expr finish_constraint_primary_expr	(cp_expr);
-extern tree finish_concept_definition		(cp_expr, tree);
+extern tree finish_concept_definition		(cp_expr, tree, tree);
 extern tree combine_constraint_expressions      (tree, tree);
 extern tree append_constraint			(tree, tree);
 extern tree get_constraints                     (const_tree);
diff --git a/gcc/cp/parser.cc b/gcc/cp/parser.cc
index e402183..c5929a6 100644
--- a/gcc/cp/parser.cc
+++ b/gcc/cp/parser.cc
@@ -29672,6 +29672,8 @@ cp_parser_concept_definition (cp_parser *parser)
       return NULL_TREE;
     }
 
+  tree attrs = cp_parser_attributes_opt (parser);
+
   if (!cp_parser_require (parser, CPP_EQ, RT_EQ))
     {
       cp_parser_skip_to_end_of_statement (parser);
@@ -29688,7 +29690,7 @@ cp_parser_concept_definition (cp_parser *parser)
      but continue as if it were.  */
   cp_parser_consume_semicolon_at_end_of_statement (parser);
 
-  return finish_concept_definition (id, init);
+  return finish_concept_definition (id, init, attrs);
 }
 
 // -------------------------------------------------------------------------- //
diff --git a/gcc/cp/pt.cc b/gcc/cp/pt.cc
index e6017b3..0310e38 100644
--- a/gcc/cp/pt.cc
+++ b/gcc/cp/pt.cc
@@ -28928,7 +28928,7 @@ placeholder_type_constraint_dependent_p (tree t)
    the TEMPLATE_DECL. */
 
 tree
-finish_concept_definition (cp_expr id, tree init)
+finish_concept_definition (cp_expr id, tree init, tree attrs)
 {
   gcc_assert (identifier_p (id));
   gcc_assert (processing_template_decl);
@@ -28962,6 +28962,9 @@ finish_concept_definition (cp_expr id, tree init)
   DECL_CONTEXT (decl) = current_scope ();
   DECL_INITIAL (decl) = init;
 
+  if (attrs)
+    cplus_decl_attributes (&decl, attrs, 0);
+
   set_originating_module (decl, false);
 
   /* Push the enclosing template.  */