c++: remove function/var concepts code

This patch removes vestigial Concepts TS code as discussed in
<https://gcc.gnu.org/pipermail/gcc-patches/2024-July/657937.html>.

In particular, it removes code related to function/variable concepts.
That includes variable_concept_p and function_concept_p, which then
cascades into removing DECL_DECLARED_CONCEPT_P etc.  So I think we
no longer need to say "standard concept" since there are no non-standard
ones anymore.

I've added two new errors saying that "variable/function concepts are
no longer supported".

gcc/cp/ChangeLog:

	* constexpr.cc (cxx_eval_constant_expression): Don't call
	unpack_concept_check.  Add a concept_check_p assert.  Remove
	function_concept_p code.
	* constraint.cc (check_constraint_atom): Remove function concepts code.
	(unpack_concept_check): Remove.
	(get_concept_check_template): Remove Concepts TS code.
	(resolve_function_concept_overload): Remove.
	(resolve_function_concept_check): Remove.
	(resolve_concept_check): Remove Concepts TS code.
	(get_returned_expression): Remove.
	(get_variable_initializer): Remove.
	(get_concept_definition): Remove Concepts TS code.
	(normalize_concept_check): Likewise.
	(build_function_check): Remove.
	(build_variable_check): Remove.
	(build_standard_check): Use concept_definition_p instead of
	standard_concept_p.
	(build_concept_check): Remove variable_concept_p/function_concept_p
	code.
	(build_concept_id): Simplify.
	(build_type_constraint): Likewise.
	(placeholder_extract_concept_and_args): Likewise.
	(satisfy_nondeclaration_constraints): Likewise.
	(check_function_concept): Remove.
	(get_constraint_error_location): Remove Concepts TS code.
	* cp-tree.h (DECL_DECLARED_CONCEPT_P): Remove.
	(check_function_concept): Remove.
	(unpack_concept_check): Remove.
	(standard_concept_p): Remove.
	(variable_concept_p): Remove.
	(function_concept_p): Remove.
	(concept_definition_p): Simplify.
	(concept_check_p): Don't check for CALL_EXPR.
	* decl.cc (check_concept_refinement): Remove.
	(duplicate_decls): Remove check_concept_refinement code.
	(is_concept_var): Remove.
	(cp_finish_decl): Remove is_concept_var.
	(check_concept_fn): Remove.
	(grokfndecl): Give an error about function concepts not being supported
	anymore.  Remove unused code.
	(grokvardecl): Give an error about variable concepts not being
	supported anymore.
	(finish_function): Remove DECL_DECLARED_CONCEPT_P code.
	* decl2.cc (min_vis_expr_r): Use concept_definition_p instead of
	standard_concept_p.
	(maybe_instantiate_decl): Remove DECL_DECLARED_CONCEPT_P check.
	(mark_used): Likewise.
	* error.cc (dump_simple_decl): Use concept_definition_p instead of
	standard_concept_p.
	(dump_function_decl): Remove DECL_DECLARED_CONCEPT_P code.
	(print_concept_check_info): Don't call unpack_concept_check.  Simplify.
	* mangle.cc (write_type_constraint): Likewise.
	* parser.cc (cp_parser_nested_name_specifier_opt): Remove
	function_concept_p code.  Only check concept_definition_p, not
	variable_concept_p/standard_concept_p.
	(add_debug_begin_stmt): Remove DECL_DECLARED_CONCEPT_P code.
	(cp_parser_template_declaration_after_parameters): Remove a stale
	comment.
	* pt.cc (check_explicit_specialization): Remove
	DECL_DECLARED_CONCEPT_P code.
	(process_partial_specialization): Remove variable_concept_p code.
	(lookup_template_variable): Likewise.
	(tsubst_expr) <case CALL_EXPR>: Remove Concepts TS code and simplify.
	(do_decl_instantiation): Remove DECL_DECLARED_CONCEPT_P code.
	(instantiate_decl): Likewise.
	(placeholder_type_constraint_dependent_p): Don't call
	unpack_concept_check.  Add a concept_check_p assert.
	(convert_generic_types_to_packs): Likewise.
	* semantics.cc (finish_call_expr): Remove Concepts TS code and simplify.

gcc/testsuite/ChangeLog:

	* g++.dg/concepts/decl-diagnose.C: Adjust dg-error.
	* g++.dg/concepts/fn-concept2.C: Likewise.
	* g++.dg/concepts/pr71128.C: Likewise.
	* g++.dg/concepts/var-concept6.C: Likewise.
	* g++.dg/cpp2a/concepts.C: Likewise.

1 files changed, 6 insertions, 54 deletions

diff --git a/gcc/cp/pt.cc b/gcc/cp/pt.cc
index 77fa590..35a9c56 100644
--- a/gcc/cp/pt.cc
+++ b/gcc/cp/pt.cc
@@ -3232,14 +3232,6 @@ check_explicit_specialization (tree declarator,
               tree tmpl_func = DECL_TEMPLATE_RESULT (gen_tmpl);
               gcc_assert (TREE_CODE (tmpl_func) == FUNCTION_DECL);
 
-              /* A concept cannot be specialized.  */
-              if (DECL_DECLARED_CONCEPT_P (tmpl_func))
-                {
-                  error ("explicit specialization of function concept %qD",
-                         gen_tmpl);
-                  return error_mark_node;
-                }
-
               /* This specialization has the same linkage and visibility as
                  the function template it specializes.  */
               TREE_PUBLIC (decl) = TREE_PUBLIC (tmpl_func);
@@ -5150,13 +5142,6 @@ process_partial_specialization (tree decl)
 
   gcc_assert (current_template_parms);
 
-  /* A concept cannot be specialized.  */
-  if (flag_concepts && variable_concept_p (maintmpl))
-    {
-      error ("specialization of variable concept %q#D", maintmpl);
-      return error_mark_node;
-    }
-
   inner_parms = INNERMOST_TEMPLATE_PARMS (current_template_parms);
   ntparms = TREE_VEC_LENGTH (inner_parms);
 
@@ -10532,9 +10517,6 @@ lookup_template_class (tree d1, tree arglist, tree in_decl, tree context,
 tree
 lookup_template_variable (tree templ, tree arglist, tsubst_flags_t complain)
 {
-  if (flag_concepts && variable_concept_p (templ))
-    return build_concept_check (templ, arglist, tf_none);
-
   tree gen_templ = most_general_template (templ);
   tree parms = DECL_INNERMOST_TEMPLATE_PARMS (gen_templ);
   arglist = add_outermost_template_args (templ, arglist);
@@ -20119,14 +20101,6 @@ tsubst_expr (tree t, tree args, tsubst_flags_t complain, tree in_decl)
 	    tree check = build_concept_check (templ, targs, complain);
 	    if (check == error_mark_node)
 	      RETURN (error_mark_node);
-
-	    tree id = unpack_concept_check (check);
-
-	    /* If we built a function concept check, return the underlying
-	       template-id. So we can evaluate it as a function call.  */
-	    if (function_concept_p (TREE_OPERAND (id, 0)))
-	      RETURN (id);
-
 	    RETURN (check);
 	  }
 
@@ -21096,19 +21070,8 @@ tsubst_expr (tree t, tree args, tsubst_flags_t complain, tree in_decl)
 	  ret = build_offset_ref_call_from_tree (function, &call_args,
 						 complain);
 	else if (concept_check_p (function))
-	  {
-	    /* FUNCTION is a template-id referring to a concept definition.  */
-	    tree id = unpack_concept_check (function);
-	    tree tmpl = TREE_OPERAND (id, 0);
-	    tree args = TREE_OPERAND (id, 1);
-
-	    /* Calls to standard and variable concepts should have been
-	       previously diagnosed.  */
-	    gcc_assert (function_concept_p (tmpl));
-
-	    /* Ensure the result is wrapped as a call expression.  */
-	    ret = build_concept_check (tmpl, args, tf_warning_or_error);
-	  }
+	  /* Calls to concepts should have been previously diagnosed.  */
+	  gcc_assert (false);
 	else
 	  ret = finish_call_expr (function, &call_args,
 				  /*disallow_virtual=*/qualified_p,
@@ -26414,14 +26377,6 @@ do_decl_instantiation (tree decl, tree storage)
       error ("explicit instantiation of non-template %q#D", decl);
       return;
     }
-  else if (DECL_DECLARED_CONCEPT_P (decl))
-    {
-      if (VAR_P (decl))
-	error ("explicit instantiation of variable concept %q#D", decl);
-      else
-	error ("explicit instantiation of function concept %q#D", decl);
-      return;
-    }
 
   bool var_templ = (DECL_TEMPLATE_INFO (decl)
                     && variable_template_p (DECL_TI_TEMPLATE (decl)));
@@ -27211,9 +27166,6 @@ instantiate_decl (tree d, bool defer_ok, bool expl_inst_class_mem_p)
      functions and static member variables.  */
   gcc_assert (VAR_OR_FUNCTION_DECL_P (d));
 
-  /* A concept is never instantiated. */
-  gcc_assert (!DECL_DECLARED_CONCEPT_P (d));
-
   gcc_checking_assert (!DECL_FUNCTION_SCOPE_P (d));
 
   if (modules_p ())
@@ -29492,8 +29444,8 @@ make_constrained_decltype_auto (tree con, tree args)
 static bool
 placeholder_type_constraint_dependent_p (tree t)
 {
-  tree id = unpack_concept_check (t);
-  tree args = TREE_OPERAND (id, 1);
+  gcc_assert (concept_check_p (t));
+  tree args = TREE_OPERAND (t, 1);
   tree first = TREE_VEC_ELT (args, 0);
   if (ARGUMENT_PACK_P (first))
     {
@@ -31452,8 +31404,8 @@ convert_generic_types_to_packs (tree parm, int start_idx, int end_idx)
          requirements.  */
       if (tree constr = TEMPLATE_PARM_CONSTRAINTS (node))
 	{
-	  tree id = unpack_concept_check (constr);
-	  TREE_VEC_ELT (TREE_OPERAND (id, 1), 0) = t;
+	  gcc_assert (concept_check_p (constr));
+	  TREE_VEC_ELT (TREE_OPERAND (constr, 1), 0) = t;
 	  /* Use UNKNOWN_LOCATION so write_template_args can tell the
 	     difference between this and a fold the user wrote.  */
 	  location_t loc = UNKNOWN_LOCATION;