How to fix strcpy so that it detects overlapping strings

Question

In an interview, I was asked to write an implementation of strcpy and then fix it so that it properly handles overlapping strings. My implementation is below and it is very naiv

Answer 1

if (a>= b && a <= b+strlen(b))) || (b+strlen(b) >= a && b+strlen(b) <= a + strlen(b))

(*) you should cache strlen(b) to improve performance

What it does:
checks if the a+len [address of a + extra len bytes] is inside the string, or a [address of a] is inside the string, these are the only possibilities for a string overlapping.