Support to disable formatters for std::optional and std::expected

[robertfalkenberg]

This MR adds support to disable formatters for std::optional and std::expected, for example, if a custom standard definition is preferred.

The MR follows the same approach as for std::variant in #3716.

To disable these formatters, define the respective macros

#define FMT_CPP_LIB_OPTIONAL 0
#define FMT_CPP_LIB_EXPECTED 0

before loading the fmt library (or via command line to the compiler).

[vitaut]

Thanks for the PR but you should only provide formatters for your own types. The macro to disable std::variant formatter is a workaround for old libc++ on macOS and will be removed.

[robertfalkenberg]

Thank you for your time reviewing the MR.

The macro to disable std::variant formatter is a workaround for old libc++ on macOS and will be removed.

OK, I agree - if the plan is to remove those workarounds, then there shouldn't be any extensions towards the opposite direction.

Do you see any other way(s) to provide a project-wide custom version for default std::optional(ANY_TYPE) that does not require

• patching of the imported fmt library and re-patching it on each update;
• wrapping each optional into a custom type with custom formatter (as proposed in Override/Replace/Customize formatter for std::optional #4452).