OverloadedStrings induces overlapping pattern matches

The OverloadedStrings extension sometimes causes GHC to warn about overlapping pattern matches, when there are not overlapping pattern matches.

To reproduce, create the file BadWarning.hs:

{-# LANGUAGE OverloadedStrings #-}
module BadWarning where

data MyString = MyString String

func (MyString "a") = undefined
func (MyString "bb") = undefined
func _ = undefined

Load the program with ghci -W BadWarning.hs, and ghci outputs

GHCi, version 7.0.2: http://www.haskell.org/ghc/  :? for help
Loading package ghc-prim ... linking ... done.
Loading package integer-gmp ... linking ... done.
Loading package base ... linking ... done.
Loading package ffi-1.0 ... linking ... done.
[1 of 1] Compiling BadWarning       ( BadWarning.hs, interpreted )

BadWarning.hs:6:1:
    Warning: Pattern match(es) are overlapped
             In an equation for `func': func (MyString "a") = ...
Ok, modules loaded: BadWarning.
*BadWarning>

The warning goes away if you remove the OverloadedStrings extension. The warning also goes away if you change (MyString "bb") to (MyString "b") or (MyString "").

Trac metadata

Trac field	Value
Version	7.0.2
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler
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