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The following program typechecks:

{-# LANGUAGE NamedWildCards #-}
{-# LANGUAGE PolyKinds #-}
{-# LANGUAGE TypeApplications #-}
module Bug where

data P (a :: k) = MkP

f :: P Int
f = MkP @_k

However, if I use a named wildcard in a visible //kind// application, such as in the following variant of f:

g :: P @_k Int
g = MkP

Then GHC errors unless I enable PartialTypeSignatures:

$ ~/Software/ghc5/inplace/bin/ghc-stage2 Bug.hs
[1 of 1] Compiling Bug              ( Bug.hs, Bug.o )

Bug.hs:11:9: error:
    • Found type wildcard ‘_k’ standing for ‘*’
      To use the inferred type, enable PartialTypeSignatures
    • In the type signature: g :: P @_k Int
   |
11 | g :: P @_k Int
   |         ^^

It feels strangely inconsistent to require the language extension in one place but not the other.