Because it's just performing a very basic type conversion that can be
done at constexpr time.
My use case simultaneously creates a
`fmt::basic_format_string<some_type_conversion<Args...>>` instance and
performs `some_type_conversion<Args>(args)...`. `some_type_conversion`
optionally applies `fmt::streamed(arg)` to a subset of types. This needs
to be `constexpr` because `basic_format_string`'s constructor is
`consteval`.
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