swift

103428fe - AST: More robust recursion check for opaque types with infinite underlying types

AST: More robust recursion check for opaque types with infinite underlying types Tracking seen declarations and substitution maps only detects the situation where the opaque type's underlying type contains itself with the same substitution map. However, it is also possible to recurse with a different substitution map. In general termination is undecidable with a problem like this, so instead of trying to catch cycles, just impose a termination limit. This converts a stack overflow into an assertion, which is still not ideal; we should really diagnose something instead. But this is a first step.