[llvm-bugs] [Bug 27225] New: accessing private field from public typedef is an error

[via llvm-bugs]

https://llvm.org/bugs/show_bug.cgi?id=27225

            Bug ID: 27225
           Summary: accessing private field from public typedef is an
                    error
           Product: clang
           Version: 3.8
          Hardware: All
                OS: All
            Status: NEW
          Severity: normal
          Priority: P
         Component: C++14
          Assignee: unassignedclangbugs at nondot.org
          Reporter: tower120 at gmail.com
                CC: llvm-bugs at lists.llvm.org
    Classification: Unclassified

The following code doesn't compiles in clang:
http://coliru.stacked-crooked.com/a/1c88b33600d369f6

template<class R, class T1, class T2 = void>
class GenericArity {
    std::integral_constant<int, 1> static get(function<R(T1)>){ return {}; }    
    std::integral_constant<int, 2> static get(function<R(T1, T2)>){ return {};
}

public:
    template<class Fn>
    using Arity = decltype(get(declval<Fn>()));

    template<class Fn>
    auto static arity(Fn&&){
        return Arity<Fn>{};
    }
};




With the following errors:

main.cpp:17:28: error: 'get' is a private member of 'GenericArity<void,
std::__cxx11::basic_string<char>, int>'
    using Arity = decltype(get(declval<Fn>()));
                           ^~~
main.cpp:46:60: note: in instantiation of template type alias 'Arity' requested
here
            using Arity = GenericArity<void, string, int>::Arity<Fn>;        
                                                           ^
main.cpp:13:43: note: implicitly declared private here
    std::integral_constant<int, 2> static get(function<R(T1, T2)>){ return {};
}
                                          ^
1 error generated.



P.S. related to https://llvm.org/bugs/show_bug.cgi?id=25334

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