`std::optional<const T>` doesn't work

[OleksandrKvl]

fmt::print("{}", std::optional<const int>{}); doesn't compile. Below is my initial analysis (I'm not really familiar with fmt internals):

• is_formattable<std::optional<const int>> is false because
• has_formatter is false because
• decltype(formatter<U, Char>().format(*p, *ctx), std::true_type()) is SFINAEd out because
• formatter<U, Char>() is invalid because
• default ctor for formatter<std::optional<const int>> is deleted because
• default ctor for formatter<T, Char> underlying_ here is deleted because
• default ctor for formatter<const int> is deleted here probably because
• native_formatter is not selected because type_constant<const int>::value is custom_type instead of int_type

I can only guess that remove_cv_t is required somewhere in that chain.

[vitaut]

Thanks for the PR and investigating the issue! It does look like remove_cv_t is missing in

fmt/include/fmt/std.h

Line 356 in d13e5d0

formatter<T, Char> underlying_;

which shouldn't be hard to fix - a PR is welcome.

Repro: https://www.godbolt.org/z/TGfobcEbb.