Add formatter for std::exception

[zach2good]

Adds a single simple formatter for std::exception and accompanying test.

Reference:
#2977
#3012

Closes #2977

[zach2good]

Will come back and fix those failing CI jobs in a couple of days 👍

[Baardi]

Sorry for a stupid, uneducated question.
But does it make sense to include the exception type as well, when formatting?

E.g. a std::runtime_error("My exception") would then be formatted as "std::runtime_error: My exception", as it typically is formatted in C#, by using typeid(xxx).name()?
Or is that what you previously discussed, and decided against it due to portability issues?

[zach2good]

Not necessarily previously discussed (@vitaut's suggestions in #3012 (comment)), but you're right in that it isn't super portable to do that:

Using:

return fmt::formatter<string_view>::format(fmt::format("{}: {}", typeid(T).name(), ex.what()), ctx);

clang (name mangling):

/mnt/c/dev/fmt/test/std-test.cc:88: Failure
Expected equality of these values:
  fmt::format("{}", ex)
    Which is: "St9exception: Test Exception"
  str
    Which is: "Test Exception"
/mnt/c/dev/fmt/test/std-test.cc:89: Failure
Expected equality of these values:
  fmt::format("{:?}", ex)
    Which is: "\"St9exception: Test Exception\""
  escstr
    Which is: "\"Test Exception\""
/mnt/c/dev/fmt/test/std-test.cc:95: Failure
Expected equality of these values:
  fmt::format("{}", ex)
    Which is: "St13runtime_error: Test Exception"
  str
    Which is: "Test Exception"
/mnt/c/dev/fmt/test/std-test.cc:96: Failure
Expected equality of these values:
  fmt::format("{:?}", ex)
    Which is: "\"St13runtime_error: Test Exception\""
  escstr
    Which is: "\"Test Exception\""

MSVC (addition of "class"):

C:\dev\fmt\test\std-test.cc(88): error: Expected equality of these values:
  fmt::format("{}", ex)
    Which is: "class std::exception: Test Exception"
  str
    Which is: "Test Exception"
C:\dev\fmt\test\std-test.cc(89): error: Expected equality of these values:
  fmt::format("{:?}", ex)
    Which is: "\"class std::exception: Test Exception\""
  escstr
    Which is: "\"Test Exception\""
C:\dev\fmt\test\std-test.cc(95): error: Expected equality of these values:
  fmt::format("{}", ex)
    Which is: "class std::runtime_error: Test Exception"
  str
    Which is: "Test Exception"
C:\dev\fmt\test\std-test.cc(96): error: Expected equality of these values:
  fmt::format("{:?}", ex)
    Which is: "\"class std::runtime_error: Test Exception\""
  escstr
    Which is: "\"Test Exception\""

[Baardi]

For gcc you can demangle the name, and for msvc you can strip the class I guess. Not sure about clang.

Searching online, it generally seems like a doable thing, at least for the 3 main compilers, although it would introduce a lot of complexity that might not not be wanted in fmtlib.

[vitaut]

Thank you!