Marge Bot pushed to branch master at Glasgow Haskell Compiler / GHC

Commits:

• bd3eba86
  by Vladislav Zavialov at 2026-02-27T05:48:01-05:00

  Check for negative type literals in the type checker (#26861)
  
  GHC disallows negative type literals (e.g., -1), as tested by T8306 and
  T8412. This check is currently performed in the renamer:
  
    rnHsTyLit tyLit@(HsNumTy x i) = do
      when (i < 0) $
        addErr $ TcRnNegativeNumTypeLiteral tyLit
  
  However, this check can be bypassed using RequiredTypeArguments
  (see the new test case T26861). Prior to this patch, such programs
  caused the compiler to hang instead of reporting a proper error.
  This patch addresses the issue by adding an equivalent check in
  the type checker, namely in tcHsType.
  
  The diff is deliberately minimal to facilitate backporting. A more
  comprehensive rework of HsTyLit is planned for a separate commit.
  

4 changed files:

• compiler/GHC/Tc/Gen/HsType.hs
• + testsuite/tests/typecheck/should_fail/T26861.hs
• + testsuite/tests/typecheck/should_fail/T26861.stderr
• testsuite/tests/typecheck/should_fail/all.T

Changes:

   = checkExpKind rn_ty liftedTypeKind liftedTypeKind exp_kind

 --------- Literals

-tcHsType _ rn_ty@(HsTyLit _ (HsNumTy _ n)) exp_kind

-  = do { checkWiredInTyCon naturalTyCon

+tcHsType _ rn_ty@(HsTyLit _ (HsNumTy x n)) exp_kind

+  = do { when (n < 0) $

+           addErr $ TcRnNegativeNumTypeLiteral (HsNumTy x n)

+       ; checkWiredInTyCon naturalTyCon

        ; checkExpKind rn_ty (mkNumLitTy n) naturalTy exp_kind }

 tcHsType _ rn_ty@(HsTyLit _ (HsStrTy _ s)) exp_kind

+{-# LANGUAGE DataKinds #-}

+{-# LANGUAGE NegativeLiterals #-}

+{-# LANGUAGE RequiredTypeArguments #-}

+

+module T26861 where

+

+import Data.Proxy

+import GHC.TypeLits

+

+main :: IO ()

+main = print (natVis (-42))

+

+natVis :: forall a -> KnownNat a => Integer

+natVis n = natVal (Proxy @n)

+T26861.hs:11:23: error: [GHC-93632]

+    • Illegal literal in type (type literals must not be negative): -42

+    • In the type ‘-42’

+      In the first argument of ‘print’, namely ‘(natVis (-42))’

+      In the expression: print (natVis (-42))

+

 test('T23162b', normal, compile_fail, [''])

 test('T23162c', normal, compile, [''])

 test('T23162d', normal, compile, [''])

+test('T26861', normal, compile_fail, [''])