Importing constructor of associated data type fails

I will base my description on the code you find when searching for "instance PersistEntity" on http://www.yesodweb.com/book/persistent. In summary, the code looks like

instance PersistEntity Person where
...
    data EntityField Person typ where
        PersonId   :: EntityField Person PersonId
        PersonName :: EntityField Person String
        PersonAge  :: EntityField Person Int
...

This code is generated from TH (see above). I have not tested if it matters that it is generated. A first step could be to test if manually replacing the TH part yields different results.

The problem is: When i add this code in one module, do not create an export list for that module, and try to import the constructors explicitly (i.e. with a list) in a different module, it does not work.

I read and tried to follow the description on https://wiki.haskell.org/GHC/Type_families, chapter 4.3.

Testcase 1

(not really expected to work; still intersting error message)

import MyModuleContainingInstancePerson ( PersonId )

ghc says

      ‘PersonId’ is a data constructor of ‘EntityField’
    To import it use
      ‘import’ MyModuleContainingInstancePerson( EntityField( VersionVersion ) )
    or
      ‘import’ MyModuleContainingInstancePerson( EntityField(..) )

Testcase 2

(no export list in MyModuleContainingInstancePerson)

import MyModuleContainingInstancePerson ( EntityField(..) )

ghc says

Module ‘MyModuleContainingInstancePerson’ does not export ‘EntityField(..)’

Testcase 3

But when i do provide an export list in MyModuleContainingInstancePerson, like

module MyModuleContainingInstancePerson ( Person(..), EntityField(..) ) where ..

Testcase 2 suddenly works.

Trac metadata

Trac field	Value
Version	7.8.4
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler
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