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

[RyanGlScott]

{-# 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.

[RyanGlScott]

This started occurring in commit 2484d4dae65c81f218dcfe494b963b2630bb8fa6 (Refactor renaming of operators/sections to fix DuplicateRecordFields bugs).

[RyanGlScott]

After skimming the commitdiff, I'm reasonably confident I know what is happening, and how fix it. I'll post a Phab Diff after I've validated.

Also, I just realized that there's another problem with the error message in GHC 8.2: it claims that a constructor :~: is in scope, but that's untrue. I'll submit a separate ticket for this later.

[RyanGlScott]

I've opened #13948 for the A data constructor of that name is in scope issue.

[Ben Gamari <ben@…>]

In 85ac65c/ghc:

Fix #13947 by checking for unbounded names more

Commit 2484d4dae65c81f218dcfe494b963b2630bb8fa6 accidentally dropped a
call to `isUnboundName` in an important location. This re-adds it.

Fixes #13947.

Test Plan: make test TEST=T13947

Reviewers: adamgundry, austin, bgamari

Reviewed By: adamgundry

Subscribers: rwbarton, thomie

GHC Trac Issues: #13947

Differential Revision: https://phabricator.haskell.org/D3718

[bgamari]

Merged to ghc-8.2 with 45588a00c06d5ee5a0fe24974f586340ab03d2de.