P0388R4 Permit conversions to arrays of unknown bound

[jensmaurer]

Fixes #2995.

[willwray]

(Not sure if this is the place for questions / clarifications. Trying anyway.)

Should the conversion from T[N] to T(&)[] be permitted for deduced type T?

C.f. GCC bug 93191
GCC has implemented P0388, so this works:

    void cat(char const(&cstr)[]) { puts(cstr); }
    cat("Hello");    // prints "Hello"

but, on changing char to auto, it is rejected:

    void cat(auto const(&cstr)[]) { puts(cstr); }
    cat("Hello");  // error: no matching function

(same with explicit template syntax template <typename C> void cat(C const(&cstr)[]);)
Ideally cat would be variadic

    void cat(auto const(&...cstr)[]) { (puts(cstr),...); }

but deduction is required for the variadic signature.