hs-boot files can have bogus declarations if they're not exported

Consider:

-- A.hs-boot
module A(foo) where
  foo :: A -> Int
  data A  = AC { bar :: Int } | CC
   -- NB: A is not exported

-- B.hs
module B where
  import {-# SOURCE #-} A

-- A.hs
module A(foo) where  
  import B
  data A = AC { foo :: Int } | BC

GHC won't complain that A is incompatibly defined, since it's not exported.

Seems relatively harmless though.

Edited Mar 10, 2019 by Simon Peyton Jones

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