C++11 operator”” with double parameter

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What is the problem?

The problem is that the Standard forbids it. Per paragraph 13.5.8./3 of the C++11 Standard on user-defined literals:

The declaration of a literal operator shall have a parameter-declaration-clause equivalent to one of the
following:

const char*
unsigned long long int
long double
char
wchar_t
char16_t
char32_t
const char*, std::size_t
const wchar_t*, std::size_t
const char16_t*, std::size_t
const char32_t*, std::size_t

Concerning a workaround, I am not sure it is needed, since the following works fine (a double gets implicitly converted to a long double, so you can pass in literals of type double):

struct str {};

str operator"" _X(long double d) {
    return str();
}

int main()
{
    str s = 4.0_X;
}

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