Extended Description

The following snippet triggers a compilation error:

#include <memory>

struct Foo {};
using FooFn = Foo ();
using FooFnPtr = Foo (*)();
using FooDel = void (*)(FooFnPtr);
static_assert(std::is_same<FooFn*, FooFnPtr>::value, "");

void del(FooFnPtr) {}
Foo fun() { return {}; }

int main() {
    std::shared_ptr<FooFn>(&fun, &del);
}

The error happens when std::shared_ptr<Foo ()> attempts to instantiate std::allocator<Foo ()> in a constructor that does not need that allocator:

In file included from prog.cc:1:
/usr/local/libcxx-head/include/c++/v1/memory:1726:45: error: multiple overloads of 'address' instantiate to the same signature 'const_pointer (const_reference) const noexcept' (aka 'Foo ((Foo (&)()) const noexcept)()')
_LIBCPP_INLINE_VISIBILITY const_pointer address(const_reference __x) const _NOEXCEPT
^
/usr/local/libcxx-head/include/c++/v1/memory:4185:44: note: in instantiation of template class 'std::__1::allocator<Foo ()>' requested here
_cntrl = new _CntrlBlk(__p, __d, allocator<_Yp>());
^
prog.cc:13:5: note: in instantiation of function template specialization 'std::__1::shared_ptr<Foo ()>::shared_ptr<Foo (), void ()(Foo (*)())>' requested here
std::shared_ptr(&fun, &del);
^

The original use case leverages std::shared_ptr to bundle a dynamically loaded function together with a deleter that unloads the dynamic library when the function is no longer referenced. It is unclear whether this use case is supported, but I did not spot anything in the standard that would forbid it.