c++: Implement C++26 P3533R2 - constexpr virtual inheritance [PR120777]

c++: Implement C++26 P3533R2 - constexpr virtual inheritance [PR120777]

The following patch implements the C++26
P3533R2 - constexpr virtual inheritance
paper.
The changes include not rejecting it for C++26, tweaking the
error wording to show that it is valid in C++26, adjusting
synthesized_method_walk not to make synthetized cdtors non-constexpr
just because of virtual base classes in C++26 and various tweaks in
constexpr.cc so that it can deal with the expressions used for
virtual base member accesses or cdtor calls which need __in_chrg
and/or __vtt_parm arguments to be passed in some cases implicitly when
they aren't passed explicitly.  And dynamic_cast constant evaluation
tweaks so that it handles also expressions with types with virtual bases.

2025-06-27  Jakub Jelinek  <jakub@redhat.com>

	PR c++/120777
gcc/
	* gimple-fold.cc (gimple_get_virt_method_for_vtable): Revert
	2018-09-18 changes.
gcc/c-family/
	* c-cppbuiltin.cc (c_cpp_builtins): Predefine
	__cpp_constexpr_virtual_inheritance=202506L for C++26.
gcc/cp/
	* constexpr.cc: Implement C++26 P3533R2 - constexpr virtual
	inheritance.
	(is_valid_constexpr_fn): Don't reject constexpr cdtors in classes
	with virtual bases for C++26, adjust error wording.
	(cxx_bind_parameters_in_call): Add ORIG_FUN argument, add
	values for __in_chrg and __vtt_parm arguments when needed.
	(cxx_eval_dynamic_cast_fn): Adjust function comment, HINT -1
	should be possible.  For C++26 if obj is cast from POINTER_PLUS_EXPR,
	attempt to use cxx_fold_indirect_ref to simplify it and if successful,
	build ADDR_EXPR of that.
	(cxx_eval_call_expression): Add orig_fun variable, set it to
	fun before looking through clones, pass it to
	cxx_bind_parameters_in_call.
	(reduced_constant_expression_p): Add SZ argument, pass DECL_SIZE
	of FIELD_DECL e.index to recursive calls and don't return false
	if SZ is non-NULL and there are unfilled fields with bit position
	at or above SZ.
	(cxx_fold_indirect_ref_1): Handle reading of vtables using
	ptrdiff_t dynamic type instead of some pointer type.  Set el_sz
	to DECL_SIZE_UNIT value rather than TYPE_SIZE_UNIT of
	DECL_FIELD_IS_BASE fields in classes with virtual bases.
	(cxx_fold_indirect_ref): In canonicalize_obj_off lambda look
	through COMPONENT_REFs with DECL_FIELD_IS_BASE in classes with
	virtual bases and adjust off correspondingly.  Remove assertion that
	off is integer_zerop, pass tree_to_uhwi (off) instead of 0 to the
	cxx_fold_indirect_ref_1 call.
	* cp-tree.h (publicly_virtually_derived_p): Declare.
	(reduced_constant_expression_p): Add another tree argument defaulted
	to NULL_TREE.
	* method.cc (synthesized_method_walk): Don't clear *constexpr_p
	if there are virtual bases for C++26.
	* class.cc (build_base_path): Compute fixed_type_p and
	virtual_access before checks for build_simple_base_path instead of
	after that and conditional cp_build_addr_expr.  Use build_simple_path
	if !virtual_access even when v_binfo is non-NULL.
	(layout_virtual_bases): For build_base_field calls use
	access_public_node rather than access_private_node if
	publicly_virtually_derived_p.
	(build_vtbl_initializer): Revert 2018-09-18 and 2018-12-11 changes.
	(publicly_virtually_derived_p): New function.
gcc/testsuite/
	* g++.dg/cpp26/constexpr-virt-inherit1.C: New test.
	* g++.dg/cpp26/constexpr-virt-inherit2.C: New test.
	* g++.dg/cpp26/constexpr-virt-inherit3.C: New test.
	* g++.dg/cpp26/feat-cxx26.C: Add __cpp_constexpr_virtual_inheritance
	tersts.
	* g++.dg/cpp2a/constexpr-dtor3.C: Don't expect one error for C++26.
	* g++.dg/cpp2a/constexpr-dtor16.C: Don't expect errors for C++26.
	* g++.dg/cpp2a/constexpr-dynamic10.C: Likewise.
	* g++.dg/cpp0x/constexpr-ice21.C: Likewise.
	* g++.dg/cpp0x/constexpr-ice4.C: Likewise.
	* g++.dg/abi/mangle1.C: Guard the test on c++23_down.
	* g++.dg/abi/mangle81.C: New test.
	* g++.dg/ipa/ipa-icf-4.C (A::A): For
	__cpp_constexpr_virtual_inheritance >= 202506L add user provided
	non-constexpr constructor.