If I explicitly specify the template argument <T> after the constructor, it builds fine with C++17 but fails with C++20 (verified with OnlineGDB).

template <class T>
struct Foo {
    Foo<T>() = default;
};

int main() {
    Foo<int> foo;
    return 0;
}

With C++20, the compiler will throw the error

main.cpp:3:16: error: expected unqualified-id before ‘)’ token
    3 |         Foo<T>() = default;
      |                ^

To build with C++20, I must remove <T>, e.g.

template <class T>
struct Foo {
    Foo() = default;
};

int main() {
    Foo<int> foo;
    return 0;
}

Why?