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Opened by Ryan Scott@RyanGlScott
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HEAD regression: TH code fails to compile due to Unit%

The following program compiles in GHC 8.8.1 and earlier, but not with GHC HEAD:

{-# LANGUAGE FlexibleContexts #-}
{-# LANGUAGE MonoLocalBinds #-}
{-# LANGUAGE TemplateHaskell #-}
module Bug where

import Language.Haskell.TH

f :: $(pure (ForallT [] [TupleT 1 `AppT` (ConT ''Show `AppT` ConT ''Int)] (ConT ''String)))
f = "abc"
$ ~/Software/ghc5/inplace/bin/ghc-stage2 Bug.hs
[1 of 1] Compiling Bug              ( Bug.hs, Bug.o, Bug.dyn_o )

Bug.hs:8:8: error:
    • Can't find interface-file declaration for type constructor or class GHC.Classes.Unit%
        Probable cause: bug in .hi-boot file, or inconsistent .hi file
        Use -ddump-if-trace to get an idea of which file caused the error
    • In the type signature: f :: (Unit Show Int => String)
  |
8 | f :: $(pure (ForallT [] [TupleT 1 `AppT` (ConT ''Show `AppT` ConT ''Int)] (ConT ''String)))
  |        ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

I'm not sure if this program should compile or not, but it does seem like we should give a better error message here.

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