derived Show instances aren't protected from RebindableSyntax+OverloadedStrings (#12688) · Issues · Glasgow Haskell Compiler / GHC

derived Show instances aren't protected from RebindableSyntax+OverloadedStrings

When using both RebindableSyntax and OverloadedStrings, deriving (Show) no longer works.

Example program:

{-# LANGUAGE RebindableSyntax, OverloadedStrings #-}

module Bug where

import Prelude (String,Show(..))

newtype Text = Text String

fromString :: String -> Text
fromString = Text

x :: Text
x = "x"

newtype Foo = Foo ()
  deriving (Show)

In this example, fromString is redefined to return a type different from [Char]. The example x typechecks due to to the RebindableSyntax and OverloadedStrings extensions. However, the generated code for the deriving (Show) instance for Foo does not typecheck, and ghc gives a code generation error:

error:
    • Couldn't match type ‘Text’ with ‘[GHC.Types.Char]’
      Expected type: String
        Actual type: Text
    • In the first argument of ‘GHC.Show.showString’, namely ‘"Foo "’
      In the first argument of ‘(GHC.Base..)’, namely
        ‘(GHC.Show.showString "Foo ")’
      In the second argument of ‘GHC.Show.showParen’, namely
        ‘((GHC.Base..) (GHC.Show.showString "Foo ") (showsPrec 11 b1))’
      When typechecking the code for ‘showsPrec’
        in a derived instance for ‘Show Foo’:
        To see the code I am typechecking, use -ddump-deriv

A solution would be to protect the code generated by deriving (Show) from OverloadedStrings and RebindableSyntax extensions.

Trac metadata

Trac field	Value
Version	8.0.1
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler
Test case
Differential revisions
BlockedBy
Related
Blocking
CC
Operating system
Architecture

When using both `RebindableSyntax` and `OverloadedStrings`, `deriving (Show)` no longer works.

Example program:

```hs
{-# LANGUAGE RebindableSyntax, OverloadedStrings #-}

module Bug where

import Prelude (String,Show(..))

newtype Text = Text String

fromString :: String -> Text
fromString = Text

x :: Text
x = "x"

newtype Foo = Foo ()
  deriving (Show)
```

In this example, `fromString` is redefined to return a type different from `[Char]`. The example `x` typechecks due to to the `RebindableSyntax` and `OverloadedStrings` extensions. However, the generated code for the `deriving (Show)` instance for Foo does not typecheck, and ghc gives a code generation error:

```hs
error:
    • Couldn't match type ‘Text’ with ‘[GHC.Types.Char]’
      Expected type: String
        Actual type: Text
    • In the first argument of ‘GHC.Show.showString’, namely ‘"Foo "’
      In the first argument of ‘(GHC.Base..)’, namely
        ‘(GHC.Show.showString "Foo ")’
      In the second argument of ‘GHC.Show.showParen’, namely
        ‘((GHC.Base..) (GHC.Show.showString "Foo ") (showsPrec 11 b1))’
      When typechecking the code for ‘showsPrec’
        in a derived instance for ‘Show Foo’:
        To see the code I am typechecking, use -ddump-deriv
```

A solution would be to protect the code generated by `deriving (Show)` from `OverloadedStrings` and `RebindableSyntax` extensions.

<details><summary>Trac metadata</summary>

| Trac field             | Value        |
| ---------------------- | ------------ |
| Version                | 8.0.1        |
| Type                   | Bug          |
| TypeOfFailure          | OtherFailure |
| Priority               | normal       |
| Resolution             | Unresolved   |
| Component              | Compiler     |
| Test case              |              |
| Differential revisions |              |
| BlockedBy              |              |
| Related                |              |
| Blocking               |              |
| CC                     |              |
| Operating system       |              |
| Architecture           |              |

</details>