Invalid private member diagnostic of template using with decltype of private member method

[llvmbot]

Bugzilla Link	42348
Version	trunk
OS	All
Reporter	LLVM Bugzilla Contributor
CC	@DougGregor,@smilingthax,@zygoloid

Extended Description

The following code fails to compile with an erroneous error "'Impl' is a private member of 'S'":

  template <typename> struct S
    {
    private:
    template <typename> static constexpr auto Impl();

    public:
    template <typename X> using U = decltype(Impl<X>());
    };

  template <typename T> template <typename>
  constexpr auto S<T>::Impl() { }

  using X = S<void>::U<void>;

The above code compiles in gcc et al. Making the following change allows compilation:

  template <typename> struct S
    {
    private:
    struct W
      {
      template <typename> static constexpr auto Impl();
      };

    public:
    template <typename X> using U = decltype(W::template Impl<X>());
    };

  template <typename T> template <typename>
  constexpr auto S<T>::W::Impl()
    { }

[llvmbot]

The following is a more minimal version of the above showing the same error:

  struct S
    {
    private:
    template <typename> static constexpr void Impl() { }

    public:
    template <typename X> using U = decltype(Impl<X>());
    };

  using X = S::U<void>;

[shafik]

Confirmed: https://godbolt.org/z/76ncerc3T

see #66604 for some more examples

[llvmbot]

@llvm/issue-subscribers-clang-frontend

Author: None (llvmbot)

| | | | --- | --- | | Bugzilla Link | [42348](https://llvm.org/bz42348) | | Version | trunk | | OS | All | | Reporter | LLVM Bugzilla Contributor | | CC | @DougGregor,@smilingthax,@zygoloid |

Extended Description

The following code fails to compile with an erroneous error "'Impl' is a private member of 'S<void>'":

  template <typename> struct S
    {
    private:
    template <typename> static constexpr auto Impl();

    public:
    template <typename X> using U = decltype(Impl<X>());
    };

  template <typename T> template <typename>
  constexpr auto S<T>::Impl() { }

  using X = S<void>::U<void>;

The above code compiles in gcc et al. Making the following change allows compilation:

  template <typename> struct S
    {
    private:
    struct W
      {
      template <typename> static constexpr auto Impl();
      };

    public:
    template <typename X> using U = decltype(W::template Impl<X>());
    };

  template <typename T> template <typename>
  constexpr auto S<T>::W::Impl()
    { }