Macro facilitating perfectly forwarded `this` pointers

I thought of a potential Fit feature today when browsing [this r/cpp thread](https://www.reddit.com/r/cpp/comments/4b7zfe/a_possible_c_standard_proposal_on_overloading/).

It would be nice sometimes to write the same function for all function qualifier overloads, especially for generic programming. The following macro, `FIT_OVERLOAD_ALL_QUALIFIERS`, facilitates this. The first argument to this macro is the user-facing member name, and the second argument is the static internal implementation, which takes a universal reference representing `*this`.

``` cpp
#include

#define FIT_OVERLOAD_ALL_DETAIL(name, impl, qual) \
template \
decltype(auto) name(Args&&... args) qual { \
using this_t = std::remove_reference_t< \
decltype(*this) \
>; \
return impl( \
static_cast(*this), \
static_cast(args)... \
); \
} \
/**/

#define FIT_OVERLOAD_ALL_QUALIFIERS(name, impl) \
FIT_OVERLOAD_ALL_DETAIL(name, impl, &) \
FIT_OVERLOAD_ALL_DETAIL(name, impl, &&) \
FIT_OVERLOAD_ALL_DETAIL(name, impl, const &) \
FIT_OVERLOAD_ALL_DETAIL(name, impl, const &&) \
FIT_OVERLOAD_ALL_DETAIL(name, impl, volatile &) \
FIT_OVERLOAD_ALL_DETAIL(name, impl, volatile &&) \
FIT_OVERLOAD_ALL_DETAIL(name, impl, const volatile &) \
FIT_OVERLOAD_ALL_DETAIL(name, impl, const volatile &&) \
/**/

//

#include

struct foo {

FIT_OVERLOAD_ALL_QUALIFIERS(operator(), print_qualifiers)

template
static void print_qualifiers(ThisRef&&) {
using no_ref = std::remove_reference_t;

if (std::is_const{}) {
std::cout << "const ";
}

if (std::is_volatile{}) {
std::cout << "volatile ";
}

if (std::is_rvalue_reference{}) {
std::cout << "&& ";
}
else if (std::is_lvalue_reference{}) {
std::cout << "& ";
}

std::cout << '\n';
}
};

int main() {
using F = foo;
using CF = const foo;
using VF = volatile foo;
using VCF = const volatile foo;

F f{};
CF cf{};
VF vf{};
VCF vcf{};

f();
cf();
vf();
vcf();

F{}();
CF{}();
VF{}();
VCF{}();
}
```

Output:

```
&
const &
volatile &
const volatile &
&&
const &&
volatile &&
const volatile &&
```

I'd be happy to submit a PR with test cases and tweaks if there is further interest here.