TypeChecking of Monad patterns incorrect with RebindableSyntax and OverloadedStrings

Using GHC 8.4.3, I'd expect the following to work:

{-# LANGUAGE RebindableSyntax, OverloadedStrings #-}

module Fail where

import Prelude hiding (fail)

foo x = do
    Just y <- x
    return y

newtype Text = Text String

fail :: Text -> a
fail (Text x) = error x

fromString :: String -> Text
fromString = Text

But it fails with:

Fail.hs:8:5-15: error:
    * Couldn't match expected type `[Char]' with actual type `Text'
    * In a stmt of a 'do' block: Just y <- x
      In the expression:
        do Just y <- x
           return y
      In an equation for `foo':
          foo x
            = do Just y <- x
                 return y
  |
8 |     Just y <- x
  |     ^^^^^^^^^^^

Given the enabled extensions, I'd expect foo to desugar as:

foo x = x >>= \v -> case v of
    Just y -> return y
    _ -> fail (fromString "pattern match error")

But looking at TcMatches.tcMonadFailOp it checks the fail operation (which is literally fail) takes an argument of type tyString (e.g. [Char]). One way around that would be to make the "fail-op" being passed around be fail . fromString if the appropriate extensions are enabled.

Trac metadata

Trac field	Value
Version	8.4.3
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler
Test case
Differential revisions
BlockedBy
Related
Blocking
CC	ndmitchell@gmail.com
Operating system
Architecture