(c-version)

there is a typo in the provided test: The output message labels both expected strings as "S2" - should be labeled as "S1" and "S2".

( JS, possibly others )

The example tests shows that the arrays are not necessarily of equal length. This should be in the description.

For C is there some guaranee that the passed arrays are allocate big enough to hold each other?

e.g. If strlen(s1) is 3 and strlen(s2) is 7, is it really OK to write 7 chars back to s1?

Seems a dangeraous thing to do, and description is unclear on this point.

I do not like that this is implemented as an extension to the Array class since the method is destructive to both the object array and the argument array.
It is really bad form for a method to alter its arguments.