C++23 support

[guwirth]

Support latest C++ standard C++23:

• Working Draft, Standard for Programming Language C++ 2023/n4944
• cppreference/C++23

does not lead to a change in grammar

C++23 feature	Paper(s)	cxx support
Narrowing contextual conversions in static_assert and constexpr if	P1401R5	YES
Removing Garbage Collection Support	P2186R2	YES
Make declaration order layout mandated	P1847R4	YES
Change scope of lambda trailing-return-type	P2036R3	YES
Relaxing some constexpr restrictions	P2448R2	YES
Simpler implicit move	P2266R3	YES
Class template argument deduction from inherited constructors	P2582R1	YES
DR20: consteval needs to propagate up	P2564R0	YES
Extending the lifetime of temporaries in range-based for loop initializer	P2718R0	YES
DR20: Adjusting the value of feature-test macro __cpp_concepts	P2493R0	YES
DR98: Relax requirements on wchar_t to match existing practices	P2460R2	YES
DR: Using unknown pointers and references in constant expressions	P2280R4	YES
DR20: The Equality Operator You Are Looking For	P2468R2	YES
DR20: char8_t Compatibility and Portability Fix	P2513R4	YES
DR: Clarify reportingof diagnostic directives and allow static_assert of non-value-dependent expressions in a template context	CWG2518	YES

changes in grammar

C++23 feature	Paper(s)	cxx support	cxx version	pull request
#warning	P2437R1	YES	v2.0	1) #2727
Support for UTF-8 as a portable source file encoding	P2295R6	YES	v2.0.2	1) #2143
#elifdef and #elifndef	P2334R1	YES	v2.1.0	#2411 #2727
if consteval	P1938R3	YES	v2.1.3	#2728
Explicit object parameter (deducing this)	P0847R7	YES	v2.1.3	#2729
Extend init-statement (of for loop) to allow alias-declaration	P2360R0	YES	v2.1.3	#2734
Multidimensional subscript operator	P2128R6	YES	v2.1.3	#2733
Attribute [[assume]]	P1774R8	YES	v2.1.3	#2732
Literal suffix for (signed) size_t	P0330R8	YES	v2.1.3	#2742
Make () more optional for lambdas	P1102R2	YES	v2.1.3	#2736
auto(x) and auto{x}	P0849R8	YES	v2.1.3	#2740
Attributes on lambdas	P2173R1	YES	v2.1.3	#2736
Labels at the end of compound statements	P2324R2	YES	v.2.1.3	#2737
static operator()	P1169R4	YES	v2.1.3	#2740
Requirements for optional extended floating-point types	P1467R9	YES	v2.1.3	#2743
static operator[]	P2589R0	YES	v2.1.3	#2740
Permitting static constexpr variables in constexpr functions=	P2647R1	YES	v2.1.3	#2740
DR20: Meaningful exports	P2615R0	YES	v2.1.3	#2738
DR11: Allow duplicate attributes	P2156R1	YES	v2.1.3	#2740
Trimming whitespaces before line splicing	P2223R2	no	todo
Non-literal variables(and labels and gotos) in constexpr functions	P2242R3	no	todo
Consistent character literal encoding	P2316R2	no	todo
Character sets and encodings	P2314R4	no	todo
Delimited escape sequences	P2290R3	no	todo
Named universal character escapes	P2071R2	no	todo
Remove non-encodable wide character literals and multicharacter wide character literals	P2362R3	no	todo
DR: C++ Identifier Syntax using Unicode Standard Annex 31	P1949R7	no	todo
Removing mixed wide string literal concatenation	P2201R1	no	todo	3)

1) already supported in previous versions
2) does not lead to a change in grammar
3) test of the current behavior necessary