C++ try not to add footguns challenge (impossible)

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

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

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

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

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

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

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

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