TIL: lerp being A*(1-t)+B*t, if you lerp between the same value, say A=50 and B=50, you'll get a value that isn't 50, for some values of t. (floats) Kinda obvious in hindsight but surprised I never hit it before. I had to put in a special case for lerp. If (A==B) return A
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I would make the argument that the base lerp function should assume a!=b, as a contract. You can assert in debug, but should impose no overhead in release. If users know their data could potentially violate this contract, they can check a==b themselves.
The idea is that the contract says “if A==B, the result is undefined”. This way, you only pay the cost of checking if you know you may need to. Some use cases may be able to guarantee A!=B and should not have to pay the overhead of checking.