c++: processing_template_decl vs template depth [PR103408]

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.