Programmer Humor

You already can do that with C++20 concepts and the requires expression

template <typename T>
concept has_member_foo = requires(T t) {
    t.foo();
};

// Will fail to instantiate (with nice error 
// message) if t.foo() is ill-formed
template <has_member_foo T>
void bar(T t) {
    // ...
}

// abbreviated form of above
void baz(has_member_foo auto t) {
    // ...
}

// verbose form of above
template <typename T> requires
    has_member_foo<T>
void biz(T t) {
    // ...
}

// same as above but with anonymous concept
template <typename T> requires
    requires(T t) { t.foo(); }
void bom(T t) {
    // ...
}

// If already inside a function
if constexpr (has_member_foo<T>) {
    // ...
}

// Same but with anonymous concept
if constexpr (requires(T t) { t.foo(); }) {
    // ...
}