presentation_type c accepted in combination with bool

[ingo-loehken]

While applying massive random tests with generated format-specs, it turned out that the c presentation-type is allowed for bool.

    std::string_view constexpr formatString{"{:X<#9c}"}; // invalid format string
    std::cout << fmt::format(formatString, false) << std::endl; // accepted, compile error expected
    std::cout << std::format(formatString, false) << std::endl; // rejected, compiler error as expected

godbolt

While writing for false the value is missing

    std::string_view constexpr formatString{"{:X<#2c}"};
    auto const s1{fmt::format(formatString, false)};
    std::cout << s1 << std::endl; // prints "X"

godbolt

for true the output seems to include uninitialized memory (unprintable chars)

    std::string_view constexpr formatString{"{:X<#2c}"};
    auto const s1{fmt::format(formatString, false)};
    std::cout << s1 << std::endl; // prints "?X", where ? is an unprintable char

godbolt

[vitaut]

The output is \0 or \1, not uninitialized, but I agree that we should forbid this as std::format does. A PR would be welcome.