static assert that template typename T is NOT complete?

Question

Is there a way to static_assert that a type T is Not complete at that point in a header? The idea is to have a compile error if someone adds #includes down

Answer 1

Here is a function using expression SFINAE based on chris proposal which allows checking whether a type is complete yet.
My adoption needs no includes, errors-out when the required argument is missing (hiding the argument was not possible) and is suitable for C++11 onward.

template
constexpr auto is_complete(int=0) -> decltype(!sizeof(T)) {
    return true;   
}

template
constexpr bool is_complete(...) {return false;}

And a test-suite:

struct S;

bool xyz() {return is_complete(0);}

struct S{};

#include 
int main() {
    std::cout << is_complete(0) << '\n';
    std::cout << xyz() << '\n';
    std::cout << is_complete(0);
}

Output:

1
0
1

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