[llvm-bugs] [Bug 27566] New: shared_ptr of function type

[via llvm-bugs]

https://llvm.org/bugs/show_bug.cgi?id=27566

            Bug ID: 27566
           Summary: shared_ptr of function type
           Product: libc++
           Version: unspecified
          Hardware: All
                OS: All
            Status: NEW
          Severity: normal
          Priority: P
         Component: All Bugs
          Assignee: unassignedclangbugs at nondot.org
          Reporter: kaballo86 at hotmail.com
                CC: llvm-bugs at lists.llvm.org, mclow.lists at gmail.com
    Classification: Unclassified

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<FooFn>(&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.

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