Class template argument deduction of member class template

Question

Consider the following

template <typename T1>
struct OuterFoo {
  template <typename T2>
  struct InnerFoo  {
    InnerFoo(T2 v2) {}
  };

  explicit OuterFoo(T1 v1) {}
};

int main() {
  OuterFoo outer_foo{6};
  OuterFoo<int>::InnerFoo inner_foo{3};

  return 0;
}

https://godbolt.org/z/RjUuWM

clang refuses to compile this, stating "no viable constructor or deduction guide for deduction of template arguments of 'InnerFoo'" - Gcc on the other hand compiles without problems.

Is the provided example correct (as by the standard) usage of CTAD and is clang at fault here?

If a deduction guide is provided, clang compiles fine, but gcc doesn't accept the guide: https://godbolt.org/z/4b-9Cr

template <typename T1>
struct OuterFoo {
  template <typename T2>
  struct InnerFoo  {
    InnerFoo(T2 v2) {}
  };

  // This Template Deduction Guide is required in clang 6
  // for the example to compile.
  // GCC compiles without this TDG but reports an error
  // with it
  // "deduction guide ‘OuterFoo<T1>::InnerFoo(T2) -> OuterFoo<T1>::InnerFoo<T2>’
  // must be declared at namespace scope"
  template<typename T2> InnerFoo(T2 v2) -> InnerFoo<T2>;

  // ...but no TDG is required for OuterFoo
  explicit OuterFoo(T1 v1) {}
};

int main() {
  OuterFoo outer_foo{6};
  OuterFoo<int>::InnerFoo inner_foo{3};

  return 0;
}

What is the state of CTAD for member template classes in clang and gcc? Is it simply not there yet?

Answer 1

What is the state of CTAD for member template classes in clang and gcc? Is it simply not there yet?

As per the language features support page for GCC , CTAD is available for versions 7 and above.

As per the language features support page for Clang , CTAD is available for versions 7 and above.

However, note that there have been defects reported and resolved in later versions of these implementations.