PR c++/91353 - P1331R2: Allow trivial default init in constexpr contexts.

PR c++/91353 - P1331R2: Allow trivial default init in constexpr contexts.

This patch implements C++20 P1331, allowing trivial default initialization in
constexpr contexts.

	* c-cppbuiltin.c (c_cpp_builtins): Adjust the value of __cpp_constexpr.

	* class.c (trivial_default_constructor_is_constexpr): Return true in
	C++20.
	* constexpr.c (cx_check_missing_mem_inits): Allow missing field
	initializers in C++20.
	(cxx_eval_call_expression): Don't clear CONSTRUCTOR_NO_CLEARING for
	constexpr constructors in C++20.
	(reduced_constant_expression_p): Don't set FIELD for union and array
	types.  Skip empty class fields without initializers.
	* decl.c (check_for_uninitialized_const_var): Permit trivial default
	initialization in constexpr.
	(next_initializable_field): Don't skip vptr fields.
	* method.c (walk_field_subobs): Still consider a constructor that
	doesn't initialize all the members constexpr.

	* g++.dg/cpp0x/constexpr-array6.C: Adjust dg-error.
	* g++.dg/cpp0x/constexpr-ctor.C: Likewise.
	* g++.dg/cpp0x/constexpr-diag3.C: Likewise.
	* g++.dg/cpp0x/constexpr-diag4.C: Likewise.
	* g++.dg/cpp0x/constexpr-ex3.C: Likewise.
	* g++.dg/cpp0x/constexpr-template2.C: Likewise.
	* g++.dg/cpp0x/constexpr-union2.C: Likewise.
	* g++.dg/cpp0x/lambda/lambda-mangle.C: Rip out a piece of code ...
	* g++.dg/cpp0x/lambda/lambda-mangle6.C: ... and put it here.
	* g++.dg/cpp0x/pr79118.C: Adjust dg-error.
	* g++.dg/cpp1y/constexpr-83921-3.C: Likewise.
	* g++.dg/cpp1y/constexpr-neg1.C: Likewise.
	* g++.dg/cpp1z/constexpr-lambda12.C: Likewise.
	* g++.dg/cpp1z/feat-cxx1z.C: Use -std=c++17.
	* g++.dg/cpp2a/constexpr-init1.C: New test.
	* g++.dg/cpp2a/constexpr-init2.C: New test.
	* g++.dg/cpp2a/constexpr-init3.C: New test.
	* g++.dg/cpp2a/constexpr-init4.C: New test.
	* g++.dg/cpp2a/constexpr-init5.C: New test.
	* g++.dg/cpp2a/constexpr-init6.C: New test.
	* g++.dg/cpp2a/constexpr-init7.C: New test.
	* g++.dg/cpp2a/constexpr-init8.C: New test.
	* g++.dg/cpp2a/constexpr-init9.C: New test.
	* g++.dg/cpp2a/constexpr-init10.C: New test.
	* g++.dg/cpp2a/constexpr-init11.C: New test.
	* g++.dg/cpp2a/constexpr-init12.C: New test.
	* g++.dg/cpp2a/constexpr-init13.C: New test.
	* g++.dg/cpp2a/constexpr-init14.C: New test.
	* g++.dg/cpp2a/constexpr-init15.C: New test.
	* g++.dg/cpp2a/constexpr-try5.C: Adjust dg-error.
	* g++.dg/cpp2a/feat-cxx2a.C: Test __cpp_constexpr.
	* g++.dg/cpp2a/lambda-mangle.C: New test.
	* g++.dg/debug/dwarf2/pr44641.C: Skip for c++2a.
	* g++.dg/ext/stmtexpr21.C: Adjust dg-error.

Co-Authored-By: Jakub Jelinek <jakub@redhat.com>
From-SVN: r279019