How to provide deduction guide for nested template class?

According to [temp.deduct.guide/3]:

(…) A deduction-guide shall be declared in the same scope as the corresponding class template and, for a member class template, with the same access. (…)

But below example doesn’t seem to compile in both [gcc] and [clang].

#include <string>

template <class>
struct Foo {
    template <class T>
    struct Bar {
        Bar(T) { }
    };
    Bar(char const*) -> Bar<std::string>;
};

int main() {
    Foo<int>::Bar bar("abc");
    static_cast<void>(bar);
}

What is the correct syntax of deduction guide for nested template class? Or maybe this one is correct but it isn’t yet supported by the compilers?


Similar syntax but without nested class compiles fine both in gcc and clang:
#include <string>

template <class T>
struct Bar {
    Bar(T) { }
};
Bar(char const*) -> Bar<std::string>;

int main() {
    Bar bar("abc");
    static_cast<void>(bar);
}

Answer

[temp.deduct.guide] includes the sentence:

A deduction-guide shall be declared in the same scope as the corresponding class template and, for a member class template, with the same access.

This suggests that your example should work – deduction guides are explicitly supported for member class templates, as long as they’re declared in the same scope and access (which would be the class scope and public – check and check).

This is gcc bug 79501 (filed by Richard Smith).