<string_view>: basic_string_view::copy() mistakenly marked constexpr before C++20 P1032R1

[crackedmind]

Describe the bug
During implementation #50 i found that string_view::copy mistakenly marked constexpr before p1032r1

Command-line test case
I guess all versions with string_view support:

C:\Users\billy\Desktop>type string_view_copy.cpp
#include <string_view>
#include <array>

constexpr auto test_string_view_copy() {
    std::string_view a("oops");
    std::array<char,3> a1 = {'1','2','3'};
    a.copy(a1.data(), 3);
    return a1.size();
}


int main() {
    constexpr auto n = test_string_view_copy();
}

C:\Users\billy\Desktop>cl /EHsc /W4 /WX .\cl /std:c++17 /EHsc /MDd /W4 /WX string_view_copy.cpp
Microsoft (R) C/C++ Optimizing Compiler Version 19.23.28106.4 for x64
Copyright (C) Microsoft Corporation.  All rights reserved.

string_view_copy.cpp
string_view_copy.cpp(13): error C2131: expression did not evaluate to a constant
C:\Program Files (x86)\Microsoft Visual Studio\2019\Community\VC\Tools\MSVC\14.23.28105\include\xstring(1322): note: failure was caused by call of undefined function or one not declared 'constexpr'
C:\Program Files (x86)\Microsoft Visual Studio\2019\Community\VC\Tools\MSVC\14.23.28105\include\xstring(1322): note: see usage of 'std::_Narrow_char_traits<char,int>::copy'

Expected behavior
Something like this

cl /std:c++17 /EHsc /MDd /W4 /WX string_view_copy.cpp
Microsoft (R) C/C++ Optimizing Compiler Version 19.23.28106.4 for x64
Copyright (C) Microsoft Corporation.  All rights reserved.

string_view_copy.cpp
string_view_copy.cpp(4): error C3615: constexpr function 'test_string_view_copy' cannot result in a constant expression
string_view_copy.cpp(7): note: failure was caused by call of undefined function or one not declared 'constexpr'
string_view_copy.cpp(7): note: see usage of 'std::basic_string_view<char,std::char_traits<char>>::copy'
string_view_copy.cpp(13): error C2131: expression did not evaluate to a constant
string_view_copy.cpp(13): note: function violates 'constexpr' rules or has errors
string_view_copy.cpp(13): note: see usage of 'test_string_view_copy'

Additional context
Add any other context about the problem here.

[StephanTLavavej]

Great catch! We should define _CONSTEXPR20 after

STL/stl/inc/yvals_core.h

Lines 445 to 450 in 447f879

	// C++17 constexpr additions
	#if _HAS_CXX17
	#define _CONSTEXPR17 constexpr
	#else // ^^^ has C++17 constexpr additions / no C++17 constexpr additions vvv
	#define _CONSTEXPR17 inline
	#endif // _HAS_CXX17

and use it in both basic_string_view::copy() and our non-Standard extension basic_string_view::_Copy_s():

STL/stl/inc/xstring

Lines 1320 to 1335 in 447f879

	constexpr size_type copy(_Out_writes_(_Count) _Elem* const _Ptr, size_type _Count, const size_type _Off = 0) const {
	// copy [_Off, _Off + Count) to [_Ptr, _Ptr + _Count)
	_Check_offset(_Off);
	_Count = _Clamp_suffix_size(_Off, _Count);
	_Traits::copy(_Ptr, _Mydata + _Off, _Count);
	return _Count;
	}
	_Pre_satisfies_(_Dest_size >= _Count) constexpr size_type _Copy_s(_Out_writes_all_(_Dest_size) _Elem* const _Dest,
	const size_type _Dest_size, size_type _Count, const size_type _Off = 0) const {
	// copy [_Off, _Off + _Count) to [_Dest, _Dest + _Count)
	_Check_offset(_Off);
	_Count = _Clamp_suffix_size(_Off, _Count);
	_Traits::_Copy_s(_Dest, _Dest_size, _Mydata + _Off, _Count);
	return _Count;
	}

[BillyONeal]

Our own _Copy_s we can make constexpr if we want in 17 mode.