Deprecate the `pattern` keyword in import and export lists

Summary

GHC Proposal #581, section 2.3, reads as follows:

Since the data specifier introduced above subsumes uses of the pattern keyword in import/export lists that are permitted under PatternSynonyms, we propose a new warning -Wpattern-namespace-specifier that warns when the pattern namespace specifier is used. Initially this warning will be added to -Wcompat.

This ticket tracks the implementation of this specific part of the proposal.

Steps to reproduce

ghci> :set -Wcompat -XPatternSynonyms
ghci> import Data.List.NonEmpty (pattern (:|))

Expected behavior

There should be a warning about the use of the pattern keyword.

Environment

• GHC version used: 9.10.1

added Pnormal Ttask ghc proposal labels

assigned to @int-index

mentioned in issue #23781

mentioned in commit edd1ca87

mentioned in merge request !14330 (closed)

mentioned in commit c7cdd4ea

mentioned in commit f4a8ce34

mentioned in commit b1212fbf

Done in b1212fbf

commit b1212fbfaf7884077386b08a4fedc1dfdc26e7fc
Author: Vladislav Zavialov <vlad.z.4096@gmail.com>
Date:   Fri May 16 18:56:13 2025 +0300

    Implement -Wpattern-namespace-specifier (#25900)
    
    In accordance with GHC Proposal #581 "Namespace-specified imports",
    section 2.3 "Deprecate use of pattern in import/export lists", the
    `pattern` namespace specifier is now deprecated.
    
    Test cases: T25900 T25900_noext

closed

Is there a plan to fix the warnings in base and in GHC itself?

We can fix them in GHC once the boot compiler is 9.14. As for base, I am not sure whether that requires CLC approval.