Internal error with PartialTypeSignatures and TH

File:

{-# LANGUAGE TemplateHaskell #-}
type T = $([t| _ |])

gives a bad error message in 8.4 and HEAD:

    • GHC internal error: ‘_’ is not in scope during type checking, but it passed the renamer
      tcl_env of environment: [r1sn :-> ATcTyCon T :: k0]
    • In the type ‘(_)’
      In the type declaration for ‘T’

Trac metadata

Trac field	Value
Version	8.4.3
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler
Test case
Differential revisions
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Related
Blocking
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Operating system
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changed milestone to %8.6.1

changed weight to 5

added Tbug Trac import labels

Trac metadata

Trac field	Value
Component	Compiler → Template Haskell

Indeed, the renamer normally catches things like type T = _, so the typechecker never has a chance to get its claws on it. This same phenomenon can also be observed in other places where wildcards are disallowed (for example, instance C (Maybe $([t| _ |])) also gives the same internal error).

added partial type sigs label

changed milestone to %8.8.1

These won't be fixed for in GHC 8.6.

changed milestone to %8.10.1

Bumping milestones of low-priority tickets.

added Pnormal label

changed milestone to %9.0.1

changed milestone to %9.2.1

This will not be fixed in time for %9.2.1

changed milestone to %9.4.1

added typed TemplateHaskell label

The error is now

[nix-shell:~/ghc-clean]$ _build/stage1/bin/ghc TTH.hs 
[1 of 2] Compiling Main             ( TTH.hs, TTH.o, TTH.dyn_o )

<no location info>: error:
    panic! (the 'impossible' happened)
  GHC version 9.3.20220204:
	tcWildCardOcc
  TcTyMode {TypeLevel, Nothing}
  _
  Call stack:
      CallStack (from HasCallStack):
        callStackDoc, called at compiler/GHC/Utils/Panic.hs:182:37 in ghc:GHC.Utils.Panic
        pprPanic, called at compiler/GHC/Tc/Gen/HsType.hs:2238:5 in ghc:GHC.Tc.Gen.HsType

Please report this as a GHC bug:  https://www.haskell.org/ghc/reportabug

removed typed TemplateHaskell label

Perhaps something that @sheaf can look into quickly.

Patch in !7644 (closed).

mentioned in commit 25d91d61

closed with commit 033e9f0f

mentioned in commit 033e9f0f