Inconsistent -Wpartial-fields warnings with (~) vs. (~~)

In this program:

{-# LANGUAGE GADTs #-}
{-# LANGUAGE TypeOperators #-}
{-# OPTIONS_GHC -Wpartial-fields #-}
module Bug where

import Data.Type.Equality

data T1 z where
  MkT1a :: { rec1 :: () } -> T1 Int
  MkT1b :: (z ~ Bool) => T1 z

data T2 z where
  MkT2a :: { rec2 :: () } -> T2 Int
  MkT2b :: (z ~~ Bool) => T2 z

I get no -Wpartial-fields–related warnings for rec1, but I do for rec2:

GHCi, version 8.6.4: http://www.haskell.org/ghc/  :? for help
Loaded GHCi configuration from /home/rgscott/.ghci
[1 of 1] Compiling Bug              ( Bug.hs, interpreted )

Bug.hs:13:14: warning: [-Wpartial-fields]
    Use of partial record field selector: ‘rec2’
   |
13 |   MkT2a :: { rec2 :: () } -> T2 Int
   |              ^^^^

This feels strangely inconsistent, since if GHC doesn't warn for rec1, which uses (~), then I feel like it shouldn't warn for rec2 either, as the only difference is that the surrounding constructor uses a slightly different equality constraint (~~).

The reason this happens is because the dataConCannotMatch function has special reasoning for (~), but not (~~). Patch incoming.

Trac metadata

Trac field	Value
Version	8.6.4
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler
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