In runtime I can:
def X(R: Any): Any = X(R)
But can't do simmilar thing for compile-time:
scala> type X[R] = X[R]
<console>:11: error: illegal cyclic reference involving type X
type X[R] = X[R]
^
Seems like infinite loop/recursion protection, but as far as I understand the Halting problem - there is no general way to detect infinite recursion for turing-complete language (as detector itself may not halt), so additional compiler's check will not generally work here.
So is there a way to get infinite recursion on scalac? And, is there any other (than preventing such recursion) reason to have illegal cyclic reference?
There is a tricky way to get StackOverflow on scalac using recursion in combination with polymorphism:
scala> trait Re { type X[R <: Re] }
warning: there were 1 feature warning(s); re-run with -feature for details
defined trait Re
scala> trait ReRe extends Re {type X[R <: Re] = R#X[R]}
defined trait ReRe
scala> type X = ReRe#X[ReRe]
java.lang.StackOverflowError
It works because compiler think that R#X[R] is called on Re, but actually ReRe was passed when calculating type X.
No idea about purpose of illegal cyclic reference - maybe it protects only from infinite loops inside compiler, but not from infinite recursion (if turing-completeness is implemented using non-tail recursion).