zonkTcTyVarToTcTyVar compiler panic with PartialTypeSignatures

See also #26030 (closed)

Summary

I ran into a compiler panic that I managed to shrink down to a problem with PartialTypeSignatures. The panicking code shouldn't compile, but it probably shouldn't panic either.

Steps to reproduce

Attempt to compile the following code with ghc 9.12.2:

{-# LANGUAGE PartialTypeSignatures #-}
module MyLib () where

fails :: _ => a
fails = id $ ()

I also have a haskell playground version where you can see the issue here: https://play.haskell.org/saved/c59WjUrS

In ghc 9.12.2 on the playground the error is

<no location info>: error:
    panic! (the 'impossible' happened)
  GHC version 9.12.2:
	zonkTcTyVarToTcTyVar
  a_awz[tyv:1]
  ()
  Call stack:
      CallStack (from HasCallStack):
        callStackDoc, called at compiler/GHC/Utils/Panic.hs:190:37 in ghc-9.12.2-5327:GHC.Utils.Panic
        pprPanic, called at compiler/GHC/Tc/Zonk/TcType.hs:277:34 in ghc-9.12.2-5327:GHC.Tc.Zonk.TcType

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

The panic goes away if I remove the partial type signature or the $.

Expected behavior

The code should fail like it does in ghc 9.10 with an error like

Main.hs:5:9: error: [GHC-25897]
    • Couldn't match expected type ‘a’ with actual type ‘()’
      ‘a’ is a rigid type variable bound by
        the inferred type of fails :: a
        at Main.hs:4:1-15
    • In the expression: id $ ()
      In an equation for ‘fails’: fails = id $ ()
    • Relevant bindings include fails :: a (bound at Main.hs:5:1)
  |
5 | fails = id $ ()
  |         ^^^^^^^

Environment

• GHC version used: 9.12.2

Optional:

• Operating System:
• System Architecture: