GHC internal error: `a' is not in scope during type checking, but it passed the renamer

In the attached code the following lines give an internal GHC error. At least this should not happen. I also think this is valid code. Removing either type annotation makes this code compile.

bug :: WrappedR Int -> SomeFunction
bug (Wrap node) = case node of
  (Memo' (emptyT :: t a b) memoize (Var m) :: R' a b Int) -> -- also breaks

/Users/alessandro/Documents/Uni/thesis/test/bug-tvar-passed-renamer.hs:14:104:
    GHC internal error: `a' is not in scope during type checking, but it passed the renamer
    tcl_env of environment: [(am4,
                              Identifier[node::R' a b Int, <NotTopLevel>, 1]),
                             (rgs,
                              Identifier[bug::WrappedR Int -> SomeFunction, <NotTopLevel>, 1])]
    In the type `R' a b Int'
    In a pattern type signature: R' a b Int
    In the pattern:
      Memo' (emptyT :: t a b)
            (memoize :: MVar (t a b) -> (a -> b) -> a -> b)
            (Var m :: R' a b Int) :: R' a b Int

Trac metadata

Trac field	Value
Version	7.6.2
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler
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