type reference used to detect recursion. Should be initialized to null.
type of 'alias this' rewrite to attempt
false if the rewrite is safe, true if it would loop back around
S0 -> (S1 -> S2 -> S3 -> S1)
S0 is not a recursive alias this, so this returns false, and a rewrite to S1 can be tried. S1 is a recursive alias this type, but since att is initialized to null, this still returns false, but att1 is set to S1. A rewrite to S2 and S3 can be tried, but when we want to try a rewrite to S1 again, we notice att == t, so we're back at the start of the loop, and this returns true.
Check and set 'att' if 't' is a recursive 'alias this' type
The goal is to prevent endless loops when there is a cycle in the alias this chain. Since there is no multiple alias this, the chain either ends in a leaf, or it loops back on itself as some point.