Interesting, I would never have guessed the test would be broken at first sight. So maybe some people enlightened in the ways of the compiler can provide a better explanation than me. Here is my guess:
#(1) is a special construct which creates a compile-time array. The compile-time makes the trick: in short the array is more or less encoded directly into the bytecodes of the method as a constant. But modifying the array with #at:put: directly modifies the bytecodes in the compiled method itself. So you create a side-effect directly into your method.
Am I the only one to find this a bit dangerous and not intuitive? Is it a well-known behavior?
I would never have guessed that in a million years. welcome to the time traveling machine
it is a bit ugly and nobody should rely on such bad behavior. Consider that it should not exist. Stef