Broken fmt::is_formattable<void>

[phprus]

Expect:

fmt::is_formattable<void>::value == false

Actual (build error):

In file included from /opt/compiler-explorer/libs/fmt/trunk/include/fmt/format.h:41,
                 from <source>:1:
/opt/compiler-explorer/libs/fmt/trunk/include/fmt/base.h: In substitution of 'template<class T, class Char> using is_formattable = fmt::v11::bool_constant<(! std::is_base_of<fmt::v11::detail::unformattable, decltype (fmt::v11::detail::arg_mapper<Char>::map(declval<T&>()))>::value)> [with T = void; Char = char]':
<source>:6:44:   required from here
/opt/compiler-explorer/libs/fmt/trunk/include/fmt/base.h:1269:73: error: forming reference to void
 1269 | using mapped_t = decltype(detail::arg_mapper<Char>::map(std::declval<T&>()));
      |                                                         ~~~~~~~~~~~~~~~~^~
Execution build compiler returned: 1

godbolt (new link):
https://godbolt.org/z/xro8619b4

[vitaut]

I think it's fine for is_formattable to give a compile-time error for void because it's not something that can be passed as an argument to a formatting function.

[phprus]

But a compile-time error breaks a possible enable_if condition like std::is_void<T>::value || is_formattable<T, Char>::value.
See: #4148

[Dani-Hub]

But a compile-time error breaks a possible enable_if condition like std::is_void<T>::value || is_formattable<T, Char>::value. See: #4148

A possible workaround in this situation is to replace the || by disjunction_v (Untested - that assumes that is_formattable is suitable as such a predicate).

[phprus]

A possible workaround in this situation is to replace the || by disjunction_v (Untested - that assumes that is_formattable is suitable as such a predicate).

No, this is not working
https://godbolt.org/z/jvqja873h

[vitaut]

Thinking more of it, other traits do work with void so let's make is_formattable work with it too.

[vitaut]

Done in 3df47a4. Thanks for the suggestion.

[Dani-Hub]

Done in 3df47a4. Thanks for the suggestion.

What about using std::is_void instead of std::is_same<void, ...>? This would also work for cv void.

[vitaut]

What about using std::is_void instead of std::is_same<void, ...>? This would also work for cv void.

A PR would be welcome.

[phprus]

A PR would be welcome.

I made this change in a separate commit in PR #4148