LWG-3554 `chrono::parse` needs `const charT*` and `basic_string_view<charT>` overloads

[HowardHinnant]

Describe the bug

A new LWG issue was just filed because of a defect in the C++20 chrono parse overloads.

https://cplusplus.github.io/LWG/issue3554

As I write this, the issue hasn't been looked at by the LWG. I am hopeful for a quick P0 resolution. The recommended wording in the issue has been implemented at https://github.com/HowardHinnant.

Command-line test case

This should work:

in >> parse("%F %T", tp);

where tp is system_clock::time_point and in is any istream. Right now it doesn't work because parse is lacking an overload to take the format string literal.

[StephanTLavavej]

I think that there may be an issue in your Proposed Resolution:

template<class charT, class traits, class Alloc, class Parsable>
  unspecified
    parse(const charT* fmt, Parsable& tp,
          basic_string<charT, traits, Alloc>& abbrev);

Effects: Equivalent to return parse(basic_string<charT, traits, Alloc>{fmt}, tp, abbrev);

This depicts using the abbrev's Alloc type for the temporary basic_string, but there's no guarantee that the user's Alloc is default-constructible. I believe that Alloc should not be used here. Possibly the traits from abbrev shouldn't be used either. (However, when the argument is basic_string_view<charT, traits> fmt, then using that traits is desirable.)

[HowardHinnant]

I agree that's an issue. LWG3554 will likely be given P2 priority and the LWG (or LEWG) can sort it out. Fwiw, no one has complained about my example implementation which has this characteristic: https://github.com/HowardHinnant/date/blob/master/include/date/date.h#L8163-L8172

I think the main thing clients will care about is that they can use string literals for the fmt argument.