Only consider version constraints with --reject-unconstrained-dependencies

[philderbeast]

Fixes #12190.

I'll squash commits before applying the merge label if this pull request is approved.

Manual QA Notes

Compare running the included package test with the version of cabal-install:exe:cabal from this branch and with a released version, such as cabal-3.16.1.0.

Run the test locally manually, it should pass.

$ ghc --numeric-version
9.14.1

$ cabal build all --enable-tests

$ cabal run cabal-testsuite:cabal-tests -- \
  --with-cabal=./dist-newstyle/build/x86_64-linux/ghc-9.14.1/cabal-install-3.19.0.0/x/cabal/build/cabal/cabal \
  cabal-testsuite/PackageTests/RequireExplicit/FlagNotVersion/cabal.test.hs --accept
  
$ echo $status
0

Run the test with a released version of cabal and it should fail:

$ cabal run cabal-testsuite:cabal-tests -- \
   --with-cabal=/home/.../.ghcup/bin/cabal \
   cabal-testsuite/PackageTests/RequireExplicit/FlagNotVersion/cabal.test.hs --accept
  
$ echo $status
1

Modify the test so that it will succeed for versions of cabal without this fix.

Tip

You may need to delete the named tests too that were added (appended) after these instructions.

$ diff --git a/cabal-testsuite/PackageTests/RequireExplicit/FlagNotVersion/cabal.test.hs b/cabal-testsuite/PackageTests/RequireExplicit/FlagNotVersion/cabal.test.hs
index 01f7429c1..4a617d2b1 100644
--- a/cabal-testsuite/PackageTests/RequireExplicit/FlagNotVersion/cabal.test.hs
+++ b/cabal-testsuite/PackageTests/RequireExplicit/FlagNotVersion/cabal.test.hs
@@ -1,4 +1,3 @@
 import Test.Cabal.Prelude
 main = cabalTest . recordMode RecordMarked $ withRepo "repo" $ do
-  res <- fails $ cabal' "v2-build" ["all", "--dry-run"]
-  assertOutputContains "not a user-provided goal" res
+  cabal "v2-build" ["all", "--dry-run"]

Run the test with a released version of cabal and observe the change in output:

$ cabal run cabal-testsuite:cabal-tests -- \
   --with-cabal=/home/.../.ghcup/bin/cabal \
   cabal-testsuite/PackageTests/RequireExplicit/FlagNotVersion/cabal.test.hs --accept

The change of output:

$ git diff
diff --git a/cabal-testsuite/PackageTests/RequireExplicit/FlagNotVersion/cabal.out b/cabal-testsuite/PackageTests/RequireExplicit/FlagNotVersion/cabal.out
index a3a5898e7..215cadb23 100644
--- a/cabal-testsuite/PackageTests/RequireExplicit/FlagNotVersion/cabal.out
+++ b/cabal-testsuite/PackageTests/RequireExplicit/FlagNotVersion/cabal.out
@@ -5,10 +5,7 @@
 Downloading the latest package list from test-local-repo
 # cabal v2-build
 Resolving dependencies...
-Error: [Cabal-7107]
-Could not resolve dependencies:
-[__0] trying: a-0 (user goal)
-[__1] next goal: some-lib (dependency of a)
-[__1] fail (not a user-provided goal nor mentioned as a constraint, but reject-unconstrained-dependencies was set)
-[__1] fail (backjumping, conflict set: a, some-lib)
-After searching the rest of the dependency tree exhaustively, these were the goals I've had most trouble fulfilling: a (2), some-lib (1)
+Build profile: -w ghc-<GHCVER> -O1
+In order, the following would be built:
+ - some-lib-1.0 (lib) (requires build)
+ - a-0 (lib) (first run)

---

Template Α: This PR modifies behaviour or interface

Include the following checklist in your PR:

• Patches conform to the coding conventions.
• Any changes that could be relevant to users have been recorded in the changelog.
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• The documentation has been updated, if necessary.
• Manual QA notes have been included.
• Tests have been added.

[philderbeast]

I don't know how, but I missed a couple of failing tests. I've fixed the test expectations now without changing the implementation:

• cabal-testsuite/PackageTests/RequireExplicit/MultiPkg/cabal.test.hs the -any constraints are not enough any more so the expectation changed for these tests to fail and I added other tests with ==1.0 constraints that pass, as before. I updated the test comments too.

• cabal-install/tests/UnitTests/Distribution/Solver/Modular/Solver.hs I split the "reject-unconstrained" test group into two for each test set of the example package database, added way more tests with more explicit expectations. Also added this comment:

  , -- reject-unconstrained-dependencies=all requires all non-goals to be
    -- version constrained. Goals don't need constraints and flag constraints
    -- are not enough.

After the test update:

:main -p "reject-unconstrained"
Unit Tests
  UnitTests.Distribution.Solver.Modular.Solver
    reject-unconstrained
      [A, B]
        accept backtracking finds all goals closed set:          OK (0.03s)
        accept non-goal 'B' version-constrained:                 OK (0.03s)
        reject non-goal 'B' flag-constrained:                    OK (0.03s)
        reject non-goal 'C' unconstrained:                       OK (0.03s)
      [base, syb, E]
        accept all goals, no other dependencies:                 OK (0.03s)
        accept non-goal 'base' version-constrained:              OK (0.03s)
        accept non-goal 'syb' version-constrained:               OK (0.03s)
        accept non-goals 'base' and 'syb' version-unconstrained: OK (0.03s)
        reject non-goal 'base' unconstrained:                    OK (0.03s)
        reject non-goal 'syb' unconstrained:                     OK (0.03s)
        reject non-goal 'base' only flag-constrained:            OK (0.03s)
        reject non-goal 'base' only stanza-constrained:          OK (0.03s)

All 12 tests passed (0.05s)

Beforehand:

:main -p "reject-unconstrained"
Unit Tests
  UnitTests.Distribution.Solver.Modular.Solver
    reject-unconstrained
      missing syb:     OK (0.03s)
      all goals:       OK (0.03s)
      backtracking:    OK (0.03s)
      failure message: OK (0.03s)

All 4 tests passed (0.05s)

[philderbeast]

@Mikolaj & @jappeace thanks for the approvals. I had to fix a couple of tests since, if you want to take another look?

[mergify]

Merge Queue Status

• ✅ Entered queue — 2026-08-22 02:34 UTC · Rule: default · triggered by rule Put pull requests in the rebase+merge queue
• ✅ Checks passed · on draft merge queue: checking master (36357f2) and #12191 together #12253
• ✅ Merged — 2026-08-22 04:28 UTC · at 4c63e7a0568ed317f37db271deab150635c772c2 · merge

This pull request spent 1 hour 54 minutes 9 seconds in the queue, including 1 hour 42 minutes 52 seconds running CI.

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