Looking at the "decompiled" version, it's clearly cubic, not exponential. There are 6 type arguments to the outer instantiation of 'Class' in the field declaration; each of those arguments is itself an instantiation of 'Class' with 6 type arguments, each of which is a third level of instantiation of 'Class' with 6 type variable references, for a total of 216 (= 6^3) type variable references.

There's nothing exponential going on here. Note that in the expression 6^3, the number that's varying is the 6, not the 3. If it were the other way around, that would be exponential.

I realize "exponential" has come into common use meaning just "growing very fast", and it's sometimes pedantic to insist on a distinction between polynomial growth and exponential growth, but in a discussion of algorithmic time complexity, we should get it right!