[#C09F005DE770948FAB82A2FA8F293C22C43C9733]
Return c converted to uppercase
constexpr char to_upper(char c) noexcept;
This function returns the character, converting it to uppercase if it is lowercase. The function is defined only for low-ASCII characters.
assert( to_upper( 'a' ) == 'A' );
Throws nothing.