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This example is a little goofy (that's why I've marked it low priority), but here goes:

-- A.hs-boot
module A where
data T
f :: T -> T

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

-- A.hs
module A(T, f) where
    import qualified B
    type T = Int
    f :: T -> T
    f x = B.f x

When compiled, we get a very interesting error:

ezyang@sabre:~$ ghc-head --make A.hs -fforce-recomp
[1 of 3] Compiling A[boot]          ( A.hs-boot, A.o-boot )
[2 of 3] Compiling B                ( B.hs, B.o )
[3 of 3] Compiling A                ( A.hs, A.o )

A.hs-boot:2:1: error:
    Type constructor ‘T’ has conflicting definitions in the module
    and its hs-boot file
    Main module: type T = Int
    Boot file:   abstract T
  |
2 | data T
  | ^^^^^^

A.hs-boot:3:1: error:
    Identifier ‘f’ has conflicting definitions in the module
    and its hs-boot file
    Main module: f :: T -> T
    Boot file:   f :: T -> T
    The two types are different
  |
3 | f :: T -> T
  | ^^^^^^^^^^^

The first error is legitimate, but the second is puzzling. I believe the problem has to do with the fact that when we load the hs-boot file for checking, we knot-tie it against itself. Thus, the T referenced in "Boot file" is the one WITHOUT the type synonym unfolding, while the main module *does* have the type synonym unfolding.

Pretty-printing is horribly confusing in these situations, so it would be nice if we could give some better guidance here.

Trac metadata

Trac field	Value
Version	8.1
Type	Bug
TypeOfFailure	OtherFailure
Priority	low
Resolution	Unresolved
Component	Compiler (Type checker)
Test case
Differential revisions
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Operating system
Architecture