Cannot decide between different `fmt::style`s at runtime, gives crashes.

[TobiSchluter]

This crashes at runtime:

#include <fmt/core.h>
#include <fmt/color.h>

int main()
{
    auto quality = [&] {
        return fmt::styled(std::string_view("great"), fmt::emphasis::bold);
    };
    auto quality2 = quality();
    bool condition = false;
    auto quality3 = [&] {
        // Doesn't compile if this is uncommented.
        //if (condition) return fmt::styled("not great", fmt::emphasis::bold);
        return fmt::styled("great", fmt::emphasis::bold);
    }();

    // Either of the following two crashes at runtime
    fmt::print("result: {}\n", quality());
    //fmt::print("result: {}\n", quality2);

    // This works, but can only work for alternatives of the same length.
    fmt::print("result: {}\n", quality3);
}

https://godbolt.org/z/3cq9eWhnM

I'm only using the string_view here because in the real world I use multiple strings of different length which end up having different types when styled and thus can't be returned from a function as for quality3. It would be great if this or something equivalent worked, nice if the limitations were documented, good if there were a compiler error and bad (for me) if I'm just too dumb. A workaround is to format the quality flag before storing it of course, but that means formatting twice.

[vitaut]

Storing the result of fmt::styled is unsafe and should be disallowed similarly to how it is done for other view-like APIs. Fixed in 5bdce18.