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Hannes Siebenhandl pushed to branch wip/fendor/26610 at Glasgow Haskell Compiler / GHC

Commits:

d54a086f

by fendor at 2026-09-07T12:56:53+02:00

Don't lose instances from shadowed imported modules with the same name

When we look for instances below the current module, we need to check
the unit id as well when we check for whether the module is the current
module, otherwise we ignore modules we import that have the same name as
the import module.

Adds regression test.

9 changed files:

+ changelog.d/26610
compiler/GHC/Driver/Env.hs
+ testsuite/tests/ghci/prog-mhu008/Makefile
+ testsuite/tests/ghci/prog-mhu008/a/M.hs
+ testsuite/tests/ghci/prog-mhu008/all.T
+ testsuite/tests/ghci/prog-mhu008/b/M.hs
+ testsuite/tests/ghci/prog-mhu008/prog-mhu008.script
+ testsuite/tests/ghci/prog-mhu008/unitA
+ testsuite/tests/ghci/prog-mhu008/unitB

Changes:

changelog.d/26610

 +section: compiler
 +synopsis: Fix instance resolution for shadowed modules
 +description:
 +  Instances imported from modules with the same module name as the importing module
 +  were dropped accidentally since we didn't account for the unit id of the imported module.
 +mrs:
 +issues: #26610

compiler/GHC/Driver/Env.hs

@@ -269,7 +269,8 @@ hugInstancesBelow hsc_env uid mnwib = do
         hugSomeThingsBelowUs (\mod_info ->
                                    let details = hm_details mod_info
                                    -- Don't include instances for the current module
 -                                  in if moduleName (mi_module (hm_iface mod_info)) == mn
 +                                  in if    moduleUnitId (mi_module (hm_iface mod_info)) == uid
 +                                        && moduleName (mi_module (hm_iface mod_info)) == mn
                                         then []
                                         else [(md_insts details, md_fam_insts details)])
                            True -- Include -hi-boot

testsuite/tests/ghci/prog-mhu008/Makefile

 +TOP=../../..
 +include $(TOP)/mk/boilerplate.mk
 +include $(TOP)/mk/test.mk

testsuite/tests/ghci/prog-mhu008/a/M.hs

 +module M (T (..)) where
++
 +newtype T = MkT Int
 +  deriving (Eq, Ord)

testsuite/tests/ghci/prog-mhu008/all.T

 +test('prog-mhu008',
 +     [extra_files(['a/', 'b/', 'unitA', 'unitB']),
 +     cmd_prefix('ghciWayFlags=' + config.ghci_way_flags),
 +     req_interp],
 +     ghci_multiunit_script, [['unitA', 'unitB'], 'prog-mhu008.script'])

testsuite/tests/ghci/prog-mhu008/b/M.hs

 +{-# LANGUAGE PackageImports #-}
++
 +module M
 +  ( module Export
 +  , cmp
 +  ) where
++
 +import "a" M as Export
++
 +cmp :: T -> T -> Bool
 +cmp x y = x <= y

testsuite/tests/ghci/prog-mhu008/prog-mhu008.script

 +:set -XPackageImports
 +import "a" M qualified as MA
 +import "b" M qualified as MB
++
 +MB.cmp (MA.T 1) (MA.T 2)

testsuite/tests/ghci/prog-mhu008/unitA

 +-this-unit-id a-0-inplace
 +-this-package-name a
 +-i
 +-ia/src
 +M

testsuite/tests/ghci/prog-mhu008/unitB

 +-this-unit-id b-0-inplace
 +-this-package-name b
 +-package-id a-0-inplace
 +-i
 +-ib/src
 +M