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Simon Jakobi pushed to branch wip/sjakobi/T27459 at Glasgow Haskell Compiler / GHC

Commits:

b85976c9

by Simon Jakobi at 2026-08-15T13:27:20+02:00

testsuite: Add determ025, an error-message determinism test

All existing tests in testsuite/tests/determinism compare interface
files; none check that error messages are independent of the order of
Uniques. Add a test that compiles a module producing an ambiguity error
with two different unique supplies and compares the errors.

Context: #27459, where lazy unifier enumeration made instance lists in
error messages potentially unique-ordered.

Assisted-by: Claude Fable 5

480e2cdf

by Simon Jakobi at 2026-08-15T14:20:28+02:00

Sort the out-of-scope instances in potential-instance lists

With -fprint-potential-instances, the "...plus N instances involving
out-of-scope types" list was printed in the enumeration order of the
instance environment, which the previous commit made
unique-dependent: the list's order varied both under
-dinitial-unique/-dunique-increment perturbation and from run to run
with --make -j. Sort it with fuzzyClsInstCmp, like the in-scope
lists.

Also extend determ025 to cover -fprint-potential-instances.

Assisted-by: Claude Fable 5

4 changed files:

compiler/GHC/Tc/Errors/Ppr.hs
+ testsuite/tests/determinism/determ025/A.hs
+ testsuite/tests/determinism/determ025/Makefile
+ testsuite/tests/determinism/determ025/all.T

Changes:

compiler/GHC/Tc/Errors/Ppr.hs

@@ -4999,10 +4999,12 @@ potentials_msg_with_options
      n_show_matches  = 3
      n_show_unifiers = 2
 -    (in_scope_matches, not_in_scope_matches) = partition inst_in_scope matches
 -    (in_scope_unifiers, not_in_scope_unifiers) = partition inst_in_scope unifiers
 -    sorted_matches = sortBy fuzzyClsInstCmp in_scope_matches
 -    sorted_unifiers = sortBy fuzzyClsInstCmp in_scope_unifiers
 +    -- Sort before partitioning so that the out-of-scope lists are also
 +    -- shown in a deterministic order (#27459).
 +    (sorted_matches, not_in_scope_matches)
 +      = partition inst_in_scope (sortBy fuzzyClsInstCmp matches)
 +    (sorted_unifiers, not_in_scope_unifiers)
 +      = partition inst_in_scope (sortBy fuzzyClsInstCmp unifiers)
      (show_these_matches, show_these_unifiers)
         | show_all_potentials = (sorted_matches, sorted_unifiers)
         | otherwise           = (take n_show_matches  sorted_matches

testsuite/tests/determinism/determ025/A.hs

 +module A where
++
 +-- An ambiguity error whose message lists potential instances, both in-scope
 +-- and (via the instance-only imports) involving out-of-scope types.
++
 +import Data.Functor.Const ()
 +import Data.Functor.Identity ()
 +import Data.Monoid ()
 +import Data.Proxy ()
 +import Data.Ord ()
++
 +v :: Int
 +v = foldr (+) 0 (pure 21)

testsuite/tests/determinism/determ025/Makefile

 +TOP=../../..
 +include $(TOP)/mk/boilerplate.mk
 +include $(TOP)/mk/test.mk
++
 +# Check that error messages don't depend on the order of Uniques (#27459):
 +# compile a module producing an ambiguity error with two unique supplies
 +# and compare the errors.
 +determ025:
 +	$(RM) A.hi A.o
 +	-'$(TEST_HC)' $(TEST_HC_OPTS) -v0 -dinitial-unique=0 -dunique-increment=1 A.hs 2> A.err.normal
 +	$(RM) A.hi A.o
 +	-'$(TEST_HC)' $(TEST_HC_OPTS) -v0 -dinitial-unique=16777215 -dunique-increment=-1 A.hs 2> A.err.reversed
 +	diff A.err.normal A.err.reversed
 +	$(RM) A.hi A.o
 +	-'$(TEST_HC)' $(TEST_HC_OPTS) -v0 -fprint-potential-instances -dinitial-unique=0 -dunique-increment=1 A.hs 2> A.err.all.normal
 +	$(RM) A.hi A.o
 +	-'$(TEST_HC)' $(TEST_HC_OPTS) -v0 -fprint-potential-instances -dinitial-unique=16777215 -dunique-increment=-1 A.hs 2> A.err.all.reversed
 +	diff A.err.all.normal A.err.all.reversed

testsuite/tests/determinism/determ025/all.T

 +test('determ025',
 +     [extra_files(['A.hs'])],
 +     makefile_test, ['determ025'])