The assert failure is triggered by the fact that you've registered your opAssign method to return the object by value.
The opAssign method should return the object by reference, i.e. "Vec2f &opAssign(const Vec2f &in)".
Still, there is a bug in the library since it should have recognized this error and reported it to you instead of entering the code path that triggered the assert failure. I'll work on fixing this.