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	YES	v2.1.3	#2749
Delimited escape sequences	P2290R3	YES	v.2.1.3	#2744
Named universal character escapes	P2071R2	YES	v.2.1.3	#2744
Non-literal variables(and labels and gotos) in constexpr functions	P2242R3	3)
Consistent character literal encoding	P2316R2	3)
Character sets and encodings	P2314R4	3)
Remove non-encodable wide character literals and multicharacter wide character literals	P2362R3	3)
DR: C++ Identifier Syntax using Unicode Standard Annex 31	P1949R7	3)
Removing mixed wide string literal concatenation	P2201R1	3)

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