Yeah, I understand. I just wanted to show that one can understand the exact nature of the approximation in 2D geometry.

I think it's useful to think of it as a separate category of "wrong" than -ffast-math, which is "wrong" because two expressions that are mathematically equivalent over real numbers, like a + (b + c) and (a + b) + c, are not equivalent over floating-point numbers.

Using lerp in place of slerp is like using x in place of sin(x). (The small-angle approximation) They're not equivalent over the real numbers.