Overloaded signatures don't seem to work or be tested

[Quuxplusone]

https://godbolt.org/z/RcyhgH

#include <cxx_function.hpp>

int foo(int);

cxx_function::function<int(), int(int)> f = foo;
cxx_function::function<int(int), int()> g = foo;

My impression from the README is that this code is supposed to compile. It doesn't.
I don't see any test cases in test/ that try to use multiple signatures, so my working hypothesis is that the README is simply over-optimistically out-of-sync with what's actually implemented.

[Quuxplusone]

...Oh wait, I mixed up my product types with my sum types. Presumably this is not supposed to compile, after all, because foo is not invokable with both signatures.

https://godbolt.org/z/-BXfP9 works OK.

int foo();
int foo(int);

cxx_function::function<int(), int(int)> f = [](auto... args) { return foo(args...); };
cxx_function::function<int(int), int()> g = [](auto... args) { return foo(args...); };

I still think there ought to be something for this in test/, though, and ideally a short example in the README.

[potswa]

There are some testcases in constructible.cpp to ensure that certain overloads compile (or, like this one, SFINAE), but of course more and deeper testing would be better and welcome.