+Note [Synonym kind loop]
+~~~~~~~~~~~~~~~~~~~~~~~~
+Notice that we eagerly grab the *kind* from the interface file, but
+build a forkM thunk for the *rhs* (and family stuff). To see why,
+consider this (Trac #2412)
+
+M.hs: module M where { import X; data T = MkT S }
+X.hs: module X where { import {-# SOURCE #-} M; type S = T }
+M.hs-boot: module M where { data T }
+
+When kind-checking M.hs we need S's kind. But we do not want to
+find S's kind from (typeKind S-rhs), because we don't want to look at
+S-rhs yet! Since S is imported from X.hi, S gets just one chance to
+be defined, and we must not do that until we've finished with M.T.
+
+Solution: record S's kind in the interface file; now we can safely
+look at it.