Clang 20 implicit CTAD regression?

[ngladitz]

#include <type_traits>

template<typename R>
struct Foo
{
	using value_type = R ;

	Foo() = default;
    Foo(const Foo&) = delete;
    Foo(Foo&&) = delete;

	Foo(Foo<Foo<R>>&& rhs)
	{
	}
};

/*template<typename R>
Foo(Foo<Foo<R>>&&) -> Foo<R>;*/

int main()
{
	auto r1 = Foo(Foo<Foo<int>>{});
	static_assert(std::is_same_v<decltype(r1)::value_type, int>);
}

Compiles with Clang 19.1.0 but fails the static_assert with Clang 20.1.0: https://godbolt.org/z/daqjan7e5
Uncommenting the explicit deduction guide seems to work around the issue.

[llvmbot]

@llvm/issue-subscribers-clang-frontend

Author: Nils Gladitz (ngladitz)

```C++ #include <type_traits>

template<typename R>
struct Foo
{
using value_type = R ;

Foo() = default;
Foo(const Foo&) = delete;
Foo(Foo&&) = delete;

Foo(Foo<Foo<R>>&& rhs)
{
}

};

/template<typename R>
Foo(Foo<Foo<R>>&&) -> Foo<R>;/

int main()
{
auto r1 = Foo(Foo<Foo<int>>{});
static_assert(std::is_same_v<decltype(r1)::value_type, int>);
}

Compiles with Clang 19.1.0 but fails the static_assert with Clang 20.1.0: https://godbolt.org/z/daqjan7e5
Uncommenting the explicit deduction guide seems to work around the issue.
</details>

[zyn0217]

Reduced to

template <typename R> struct Foo {
  using value_type = R;

  Foo() = default;
  Foo(Foo<Foo<R>> &&rhs) {}
};

int main() {
  auto r1 = Foo(Foo<Foo<int>>{});
  static_assert(__is_same(decltype(r1)::value_type, int));
}

https://godbolt.org/z/9ncKnrjb1

Both 19 and 20 produce the very same deduction guides:

template <typename R> Foo(Foo<Foo<R>> &&rhs) -> Foo<R>
template <typename R> Foo(Foo<R>) -> Foo<R>
template <typename R> Foo() -> Foo<R>

template <typename R> Foo(Foo<Foo<R>> &&rhs) -> Foo<R>
template <typename R> Foo(Foo<R>) -> Foo<R>
template <typename R> Foo() -> Foo<R>