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Consider

{-# LANGUAGE PartialTypeSignatures #-}

f :: _ -> _
f x = x

With GHC 8.6 we got

T16152.hs:10:6: warning: [-Wpartial-type-signatures]
    • Found type wildcard ‘_’ standing for ‘w’
      Where: ‘w’ is a rigid type variable bound by
               the inferred type of f :: w -> w
               at T16152.hs:11:1-7
    • In the type signature: f :: _ -> _
   |
10 | f :: _ -> _
   |      ^

T16152.hs:10:11: warning: [-Wpartial-type-signatures]
    • Found type wildcard ‘_’ standing for ‘w’
      Where: ‘w’ is a rigid type variable bound by
               the inferred type of f :: w -> w
               at T16152.hs:11:1-7
    • In the type signature: f :: _ -> _
   |
10 | f :: _ -> _
   |           ^

But with HEAD we get

T16152.hs:10:6: warning: [-Wpartial-type-signatures]
    • Found type wildcard ‘_’ standing for ‘_’
      Where: ‘_’ is a rigid type variable bound by
               the inferred type of f :: _ -> _
               at T16152.hs:11:1-7
    • In the type ‘_ -> _’
      In the type signature: f :: _ -> _
   |
10 | f :: _ -> _
   |      ^

T16152.hs:10:11: warning: [-Wpartial-type-signatures]
    • Found type wildcard ‘_’ standing for ‘_’
      Where: ‘_’ is a rigid type variable bound by
               the inferred type of f :: _ -> _
               at T16152.hs:11:1-7
    • In the type ‘_ -> _’
      In the type signature: f :: _ -> _
   |
10 | f :: _ -> _
   |           ^

Saying "Found type wildcard ‘’ standing for ‘’" is unhelpful. The "w" form is much better.

The change is caused by

commit 17bd163566153babbf51adaff8397f948ae363ca
Author: mynguyen <mnguyen1@brynmawr.edu>
Date:   Tue Dec 18 11:52:26 2018 -0500

    Visible kind application

which has this patch

-newWildTyVar _name
+newWildTyVar
   = do { kind <- newMetaKindVar
        ; uniq <- newUnique
        ; details <- newMetaDetails TauTv
-       ; let name = mkSysTvName uniq (fsLit "w")
+       ; let name = mkSysTvName uniq (fsLit "_")
              tyvar = (mkTcTyVar name kind details)
        ; traceTc "newWildTyVar" (ppr tyvar)
        ; return tyvar }

What was the reason for this change? Can we change "_" back to "w" please?

Trac metadata

Trac field	Value
Version	8.6.3
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler
Test case
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