libc++ on Apple platforms may not provide floating-point std::from_chars

[zhjwpku]

It seems that on macOS, when using Clang with libc++, linking fails with call to deleted function 'from_chars' for floating-point types. I created a Compiler Explorer example to demonstrate the issue:

https://godbolt.org/z/3oar38qG9

The error indicates that std::from_chars is only enabled for integral types in this libc++ version, and the floating-point overload is not available. (clang 20 supported from_chars)

Given this, I'm wondering whether we should fall back to std::strtof / std::strtod / std::strtold when parsing floating-point numbers. What do you think?

[HuaHuaY]

It seems that macOS only supports it in macOS 26's SDK.