`<charconv>`: Make `_Digit_from_byte` a function-local `static constexpr` variable again

[frederick-vs-ja]

Currently, _Digit_from_byte is not a function-local static constexpr variable although it's only used by one function.

STL/stl/inc/charconv

Lines 247 to 266 in 020513e

	// convert ['0', '9'] ['A', 'Z'] ['a', 'z'] to [0, 35], everything else to 255
	inline constexpr unsigned char _Digit_from_byte[] = {255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255,
	255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255,
	255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 255, 255, 255, 255, 255,
	255, 255, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30, 31, 32, 33, 34, 35,
	255, 255, 255, 255, 255, 255, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30,
	31, 32, 33, 34, 35, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255,
	255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255,
	255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255,
	255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255,
	255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255,
	255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255, 255};
	_STL_INTERNAL_STATIC_ASSERT(_STD size(_Digit_from_byte) == 256);
	_NODISCARD _CONSTEXPR23 unsigned char _Digit_from_char(const char _Ch) noexcept {
	// convert ['0', '9'] ['A', 'Z'] ['a', 'z'] to [0, 35], everything else to 255
	// CodeQL [SM01954] This index is valid: we cast to unsigned char and the array has 256 elements.
	return _Digit_from_byte[static_cast<unsigned char>(_Ch)];
	}

Long time ago, _Digit_from_byte was a static constexpr variable. However, due to the restriction of constant evaluation before WG21-P2647R1, #3049 moved it to namespace scope to make _Digit_from_char satisfy the restrictions of constexpr functions.

As WG21-P2647R1 was eventually adopted for C++23 and supported compilers that support C++23 should have it implemented, it should be OK to make _Digit_from_byte function-local again.

• For CUDA, it seems that CUDA 13.3.0 added support for C++23 and WG21-P2647R1 together. So we won't be blocked on CUDA if we're fortunate enough.

This seems to be a good first issue Good for newcomers .

[jovibor]

If you make it local static constexpr variable, how will it compile in modes prior to C++23?
WG21-P2647R1 is only for C++23, right?

[frederick-vs-ja]

If you make it local static constexpr variable, how will it compile in modes prior to C++23? WG21-P2647R1 is only for C++23, right?

_Digit_from_char is marked constexpr only since C++23 so I think everything will be fine.

[jovibor]

Oh, now I see this subtlety, I missed that.
static constexpr was always ok in non-constexpr functions. It didn't work only in constexpr ones.
But now it's fixed, since C++23.