Fix noexcept specialization on function pointers

[mmha]

I just (indirectly) tried to use your library. Unfortunately, the specialization on noexcept of function pointers (#89) does not work on clang:

template<typename T>
struct S;

template<bool IsNoexcept>
struct S<void(*)() noexcept(IsNoexcept)> {
	S() {}
};

void f() {}

int main() {
	S<decltype(&f)> {};
}

Clang output:

struct S<void(*)() noexcept(IsNoexcept)> {
       ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
example.cpp:4:15: note: non-deducible template parameter 'IsNoexcept'
template<bool IsNoexcept>
              ^
example.cpp:12:2: error: implicit instantiation of undefined template 'S<void (*)()>'
        S<decltype(&f)> {};
        ^
example.cpp:2:8: note: template is declared here
struct S;
       ^

In fact, just including callable_traits causes clang to emit those errors in C++17 mode. The same code crashes gcc, removing the constructor is a workaround and results in a successful compilation.

I cannot back it up with a reference to the standard, but the clang implementation suggests that this is not legal C++17. I don't trust the gcc implementation in that regard because of the bug I found.

[badair]

Thank you for the report; this is quite unfortunate. I asked about this here. libstdc++ uses this technique for its definition of is_function, which makes me think this is a QOI issue that I'll need to work around.

Relates to #85

[badair]

I used this technique to cut down on the amount of preprocessed text in half when compiling with c++1z. I'll start working to revert this optimization, but it will increase the preprocessed LOC from 20k to 40k :(

[badair]

closed via e310b2b. Bumped version to 2.0.0 which includes all post-review changes so far.