GHC panics if type in .hs-boot file has greater arity than in the .hs file

Compiling these two mutually-recursive modules triggers a panic:

-- A.hs-boot
module A where

data Blah a b

-- A.hs
module A where
import {-# SOURCE #-} B

data Blah a

foo :: String -> Blah ()
foo = bar

-- B.hs-boot
module B where
import {-# SOURCE #-} A

bar :: String -> Blah () ()

module B where
import A

bar :: String -> Blah () ()
bar = undefined

$ ghc -c --make A.hs
[1 of 4] Compiling A[boot]          ( A.hs-boot, A.o-boot )
[2 of 4] Compiling B[boot]          ( B.hs-boot, B.o-boot )
[3 of 4] Compiling A                ( A.hs, A.o )
ghc: panic! (the 'impossible' happened)
  (GHC version 8.10.1:
	piResultTys1
  *
  [()]
  Call stack:
      CallStack (from HasCallStack):
        callStackDoc, called at compiler/utils/Outputable.hs:1179:37 in ghc:Outputable
        pprPanic, called at compiler/types/Type.hs:1104:5 in ghc:Type

The program should be rejected because the declaration of Blah in A.hs-boot has two type parameters while in A.hs it only has one. But GHC should obviously not panic.