QtConcurrent::run requires the return type to be default-constructible

The issue is as simple as

class X { public: X(int) { } };

void f() {
   QFuture<X> x = QtConcurrent::run([]{ return X(0); });
}

It fails with a very long error message! In there we can find

error: use of deleted function ‘QtConcurrent::RunFunctionTask<X>::RunFunctionTask()" ... "error: no matching function for call to ‘X::X()"

Apparently, QtConcurrent::run wants to save the lambdas' return value in a member and later assign it, but if the value is not default constructible, that won't work!