Added format_as for std::vector<bool>::reference

[felix642]

Added a format_as entry to support std::vector<bool>::reference.

closes #3567

[vitaut]

Thanks for the PR. Please use format_as instead of a macro and move this to fmt/std.h.

[felix642]

Hi  @vitaut, thank you for the feedback. I've moved the format_as method to the std.h file and changed the macro for a template.

From what I can see I need to inject this specialization into the standard namespace which is probably not something that we want. I also saw this comment in the code for another specialization:
// This is defined in core.h instead of format.h to avoid injecting in std.

Do we want to do the same with this method ?

[vitaut]

You are right, injecting format_as into the std namespace is not a good idea. Let's define a formatter specialization that derives from formatter<bool> instead.

[phprus]

@vitaut
We can't implement formatter for std::vector<bool>::reference without fix issue #2979 because std::vector<bool>::reference has an overloaded operator&:

/Users/phprus/Devel/fmt/include/fmt/core.h:1286:20: error: const_cast from '__bit_iterator<std::vector<bool>, false>' to 'value_type *' (aka 'std::__bit_reference<std::vector<bool>, true> *') is not allowed custom.value = const_cast<value_type*>(&val);

[phprus]

Another solution: #3570

[vitaut]

Closing in favor of #3570 but thanks for your work!