Incorrect unused-import warning in ghci

[Berengal]

Try to load the following in ghci with -fwarn-unused-imports:

import Prelude hiding (zip)

zip :: [a] -> [b] -> [(a,b)]
zip _ _ = []

It spits out the usual warning for unused imports:

    Warning: The import of `Prelude' is redundant
               except perhaps to import instances from `Prelude'
             To import instances alone, use: import Prelude()

When compiled with ghc --make -fwarn-unused-imports no warning is printed. Also, the following does not generate a warning in ghci:

import Prelude hiding (zip)

zip :: [a] -> [b] -> [(a,b)]
zip _ _ = undefined

I expect ghci and ghc to agree on the warnings unless there's a good reason not to, and I do not expect warnings about declarations to depend on their implementation.

[simonpj]

I believe you may be misled by two things:

• ghc --make does not recompile things that have already been compiled, and that can lead to lack of warnings if you do ghc --make right after (say) your ghci run. Try adding -fforce-recomp

• undefined is not a keyword: it is simply a named definition that is indeed imported from the Prelude. So there really is a difference between [] and undefined on the RHS of your definition.

Everything seems to be working fine to me. If you still thing there's a bug, by all means re-open this.