GHC accepts import of private data constructor if it has the same name as the type
$ cat A.hs B.hs module A (T) where data T = T module B where import A (T(T))
ghci-7.8.2 accepts module B, whereas
B.hs:3:11: Module `A' does not export `T(T)'
For this bug to happen it is important, that the data constructor has the same name as the type.
I.e. if you rename the data constructor to, say,
ghci-7.8.2 will emit the same error as earlier versions.
The bug is not dramatic, since if you try to actually use the imported data constructor then ghc will say
B.hs:20:5: Not in scope: data constructor ‘T’
However this error message is very confusing, because it suggests that
T is not in scope although you imported it correctly.