c++: processing_template_decl vs template depth [PR103408]

We use processing_template_decl in two slightly different ways: as a
flag to signal that we're dealing with templated trees, and as a measure
of the current syntactic template nesting depth.  This overloaded
meaning of p_t_d is conceptually confusing and leads to bugs that we end
up working around in an ad-hoc fashion.

This patch replaces all uses of processing_template_decl that care about
its magnitude to instead look at the depth of current_template_parms
via a new macro current_template_depth.  This allows us to eliminate 3
workarounds in the concepts code: two about non-templated
requires-expressions (in constraint.cc) and one about lambdas inside
constraints (in cp_parser_requires_clause_expression etc).  This also
fixes the testcase in PR103408 about auto(x) used inside a non-templated
requires-expression.

The replacement was mostly mechanical, aside from two issues:

  * In synthesize_implicit_template_parm, when introducing a new template
    parameter list for an abbreviated function template, we need to add
    the new level of current_template_parms sooner, before calling
    process_template_parm, since this latter function now looks at
    current_template_depth to determine the level of the new parameter.

  * In instantiate_class_template_1 after substituting a template
    friend declaration, we currently increment processing_template_decl
    around the call to make_friend_class so that the friend_depth
    computation within this subroutine yields a nonzero value.  We could
    just replace this with an equivalent manipulation of
    current_template_depth, but this patch instead rewrites the
    friend_depth calculation within make_friend_class to not depend on
    p_t_d / c_t_d at all when called from instantiate_class_template_1.

	PR c++/103408

gcc/cp/ChangeLog:

	* constraint.cc (type_deducible_p): Remove workaround for
	non-templated requires-expressions.
	(normalize_placeholder_type_constraints): Likewise.
	* cp-tree.h (current_template_depth): Define.
	(PROCESSING_REAL_TEMPLATE_DECL): Inspect current_template_depth
	instead of the magnitude of processing_template_decl.
	* decl.c (start_decl): Likewise.
	(grokfndecl): Likewise.
	(grokvardecl): Likewise.
	(grokdeclarator): Likewise.
	* friend.c (make_friend_class): Likewise.  Calculate
	friend_depth differently when called at instantiation time
	instead of parse time.
	(do_friend): Likewise.
	* parser.c (cp_parser_requires_clause_expression): Remove
	workaround for lambdas inside constraints.
	(cp_parser_constraint_expression): Likewise.
	(cp_parser_requires_expression): Likewise.
	(synthesize_implicit_template_parm): Add to current_template_parms
	before calling process_template_parm.
	* pt.c (inline_needs_template_parms): Inspect
	current_template_depth instead of the magnitude of
	processing_template_decl.
	(push_inline_template_parms_recursive): Likewise.
	(maybe_begin_member_template_processing): Likewise.
	(begin_template_parm_list): Likewise.
	(process_template_parm): Likewise.
	(end_template_parm_list): Likewise.
	(push_template_decl): Likewise.
	(add_inherited_template_parms): Likewise.
	(instantiate_class_template_1): Don't adjust
	processing_template_decl around the call to make_friend_class.
	adjust_processing_template_decl to adjust_template_depth.  Set
	current_template_parms instead of processing_template_decl when
	adjust_template_depth.
	(make_auto_1): Inspect current_template_depth instead of the
	magnitude of processing_template_decl.
	(splice_late_return_type): Likewise.
	* semantics.c (fixup_template_type): Likewise.

gcc/testsuite/ChangeLog:

	* g++.dg/concepts/diagnostic18.C: Expect a "constraints on a
	non-templated function" error.
	* g++.dg/cpp23/auto-fncast11.C: New test.

1 files changed, 11 insertions, 17 deletions

diff --git a/gcc/cp/parser.c b/gcc/cp/parser.c
index 52225d4..c2564e5 100644
--- a/gcc/cp/parser.c
+++ b/gcc/cp/parser.c
@@ -29779,14 +29779,9 @@ static tree
 cp_parser_requires_clause_expression (cp_parser *parser, bool lambda_p)
 {
   processing_constraint_expression_sentinel parsing_constraint;
-  temp_override<int> ovr (processing_template_decl);
-  if (!processing_template_decl)
-    /* Adjust processing_template_decl so that we always obtain template
-       trees here.  We don't do the usual ++processing_template_decl
-       because that would skew the template parameter depth of a lambda
-       within if we're already inside a template.  */
-    processing_template_decl = 1;
+  ++processing_template_decl;
   cp_expr expr = cp_parser_constraint_logical_or_expression (parser, lambda_p);
+  --processing_template_decl;
   if (check_for_bare_parameter_packs (expr))
     expr = error_mark_node;
   return expr;
@@ -29805,12 +29800,10 @@ static tree
 cp_parser_constraint_expression (cp_parser *parser)
 {
   processing_constraint_expression_sentinel parsing_constraint;
-  temp_override<int> ovr (processing_template_decl);
-  if (!processing_template_decl)
-    /* As in cp_parser_requires_clause_expression.  */
-    processing_template_decl = 1;
+  ++processing_template_decl;
   cp_expr expr = cp_parser_binary_expression (parser, false, true,
 					      PREC_NOT_OPERATOR, NULL);
+  --processing_template_decl;
   if (check_for_bare_parameter_packs (expr))
     expr = error_mark_node;
   expr.maybe_add_location_wrapper ();
@@ -29924,11 +29917,9 @@ cp_parser_requires_expression (cp_parser *parser)
       parms = NULL_TREE;
 
     /* Parse the requirement body. */
-    temp_override<int> ovr (processing_template_decl);
-    if (!processing_template_decl)
-      /* As in cp_parser_requires_clause_expression.  */
-      processing_template_decl = 1;
+    ++processing_template_decl;
     reqs = cp_parser_requirement_body (parser);
+    --processing_template_decl;
     if (reqs == error_mark_node)
       return error_mark_node;
   }
@@ -48091,6 +48082,10 @@ synthesize_implicit_template_parm  (cp_parser *parser, tree constr)
   gcc_assert(!proto || TREE_CODE (proto) == TYPE_DECL);
   synth_tmpl_parm = finish_template_type_parm (class_type_node, synth_id);
 
+  if (become_template)
+    current_template_parms = tree_cons (size_int (current_template_depth + 1),
+					NULL_TREE, current_template_parms);
+
   /* Attach the constraint to the parm before processing.  */
   tree node = build_tree_list (NULL_TREE, synth_tmpl_parm);
   TREE_TYPE (node) = constr;
@@ -48130,8 +48125,7 @@ synthesize_implicit_template_parm  (cp_parser *parser, tree constr)
 
       tree new_parms = make_tree_vec (1);
       TREE_VEC_ELT (new_parms, 0) = parser->implicit_template_parms;
-      current_template_parms = tree_cons (size_int (processing_template_decl),
-					  new_parms, current_template_parms);
+      TREE_VALUE (current_template_parms) = new_parms;
     }
   else
     {