Misleading GHCi errors when package is installed

When a package is installed in a cabal sandbox/global package directory and code is loaded with GHCi when a file has been deleted, GHCi gives very misleading errors.

First, if we don't install the package, we get the correct error/expected result:

> mv B.hs B.hs.old
> ghci Main.hs
Main.hs:3:1: error:
    Failed to load interface for ‘B’
    Use -v to see a list of the files searched for.
Failed, modules loaded: A.

But if we first install the package, we get a very strange error:

> mv B.hs.old B.hs
> cabal install
> mv B.hs B.hs.old
> ghci Main.hs
Main.hs:6:14: error:
    Ambiguous occurrence ‘str’
    It could refer to either ‘A.str’,
                             imported from ‘A’ at Main.hs:2:1-8
                             (and originally defined at A.hs:7:1-3)
                          or ‘B.str’,
                             imported from ‘B’ at Main.hs:3:1-8 (and originally defined in ‘A’)
Failed, modules loaded: A.

I'm using GHC 8.0.1, cabal-install-1.24.0.0, and Cabal Library-1.24.0.0.

This is Very Annoying because I use this process to minimize examples. For example, when I want to simplify code by removing a module, I just delete it and then rely on GHCi to tell me what I need to update. Anyone not aware of this bug would be totally confused by the error, and even though I do know about the bug, the error is completely unhelpful.

Trac metadata

Trac field	Value
Version	8.0.1
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	GHCi
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