[llvm-bugs] [Bug 37617] New: Some casts to base class conversion are listed for overload resolution& template parameter deduction even though they are inaccessible

[via llvm-bugs]

https://bugs.llvm.org/show_bug.cgi?id=37617

            Bug ID: 37617
           Summary: Some casts to base class conversion are listed for
                    overload resolution& template parameter deduction
                    even though they are inaccessible
           Product: clang
           Version: 6.0
          Hardware: PC
                OS: Windows NT
            Status: NEW
          Severity: normal
          Priority: P
         Component: C++11
          Assignee: unassignedclangbugs at nondot.org
          Reporter: janos.boudet at nerd.nintendo.com
                CC: dgregor at apple.com, llvm-bugs at lists.llvm.org

//The bug happens since 3.9.0 until at least 6.0.0 

//declaration
template<typename... Ts> class A;

//specialization for > 0 params
template<typename T0, typename... Ts>
class A<T0, Ts...> : private A<Ts...>{};
//specialization for 0 params
template<> class A<>{};


template<typename... Ts>
void foo(A<Ts...> const&);

class B : public A<int, float>{};

int main() {

    B b;
    foo(b);


    //it seems that there's an (erroneous?) ambiguity between
    //1. foo(A<int, float>)
    //2. foo(A<float>)
    //3. foo(A<>)
    //I expected 2 and 3 to be ignored (inaccessible), because they
    //would require a reference cast to a privately inherited base type,
    //and thus 1 be the only choice, with no ambiguity
    //
    //removing the inheritance in the >0 param specialization of A removes 
    //the error

    return 0;
}

// compiler error clang 6.0.0
// <source>:19:5: error: no matching function for call to 'foo'
//
//    foo(b);
//
//    ^~~
//
// <source>:12:6: note: candidate template ignored: failed template argument
deduction
//
//void foo(A<Ts...> const&);

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