GHC 8.2 gives misleading error message for out-of-scope infix type constructor

{-# LANGUAGE TypeOperators #-}
module Bug where

f :: () -> Int :~: Int
f = undefined

GHC 8.0.2 gives a reasonable error message:

$ /opt/ghc/8.0.2/bin/ghc Bug.hs
[1 of 1] Compiling Bug              ( Bug.hs, Bug.o )

Bug.hs:4:12: error: Not in scope: type constructor or class ‘:~:’

But GHC 8.2.1's error message is much more confusing:

$ /opt/ghc/8.2.1/bin/ghc Bug.hs
[1 of 1] Compiling Bug              ( Bug.hs, Bug.o )

Bug.hs:4:6: error:
    Precedence parsing error
        cannot mix ‘(->)’ [infixr 0] and ‘:~:’ [infixl 0] in the same infix expression
  |
4 | f :: () -> Int :~: Int
  |      ^^^^^^^^^^^^^^^^^

Bug.hs:4:12: error:
    Not in scope: type constructor or class ‘:~:’
    A data constructor of that name is in scope; did you mean DataKinds?
  |
4 | f :: () -> Int :~: Int
  |            ^^^^^^^^^^^

I was thrown akilter by the claim that :~: was infixl 0, since it was displayed before the real cause of the error (that it's out-of-scope). Adding import Data.Type.Equality fixes the error, but GHC really shouldn't be making claims about the fixities of out-of-scope type constructors in the first place.

Trac metadata

Trac field	Value
Version	8.2.1-rc2
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler
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