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HEAD regression (tcMatches: multiple arguments in EmptyCase) (#25960) · Issues · Glasgow Haskell Compiler / GHC · GitLab
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HEAD regression (tcMatches: multiple arguments in EmptyCase)
Summary
GHC HEAD (as of commit 5977c6a1) panics on a valid program that successfully compiles with GHC 9.12 or earlier.
Steps to reproduce
Compile this program:
{-# LANGUAGE GHC2021 #-}
{-# LANGUAGE LambdaCase #-}
module Bug where
import Data.Void (Void)
f :: (forall a. Void -> a) -> (forall a. Void -> a)
f g = g
absurd :: Void -> a
absurd = f (\case)
-- absurd = \case -- Doesn't panicOn GHC 9.12 or earlier this succeeds, but with GHC HEAD, this fails with:
$ ~/Software/ghc-9.13.20250411/bin/ghc Bug.hs
[1 of 1] Compiling Bug ( Bug.hs, Bug.o )
<no location info>: error:
panic! (the 'impossible' happened)
GHC version 9.13.20250411:
tcMatches: multiple arguments in EmptyCaseThis regression was most likely introduced in commit cce869ea (Error message with EmptyCase and RequiredTypeArguments (#25004)). cc @int-index
Expected behavior
The program continues to compile successfully using GHC HEAD.
Environment
- GHC version used: HEAD (commit 5977c6a1)
Optional:
- Operating System: Ubuntu 22.04
- System Architecture: x86-64
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