haskell · philderbeast · Jul 30, 2026 · Aug 13, 2026
diff --git a/Cabal-syntax/src/Distribution/Version.hs b/Cabal-syntax/src/Distribution/Version.hs
@@ -108,18 +108,25 @@ import Distribution.Types.VersionRange
 -- | This is the converse of 'isAnyVersion'. It check if the version range is
 -- empty, if there is no possible version that satisfies the version range.
 --
--- For example this is @True@ (for all @v@):
+-- For example, @\<v && >v@ is no version for all @v@.
 --
--- > isNoVersion (EarlierVersion v `IntersectVersionRanges` LaterVersion v)
+-- >>> ordNub [isNoVersion (earlierVersion v `intersectVersionRanges` laterVersion v) | v <- mkVersion <$> [[0],[1]]]
+-- [True]
+--
+-- >>> isNoVersion <$> [noVersion, anyVersion]
+-- [True,False]
 isNoVersion :: VersionRange -> Bool
 isNoVersion vr = case asVersionIntervals vr of
   [] -> True
   _ -> False
 
 -- | Is this version range in fact just a specific version?
 --
--- For example the version range @\">= 3 && <= 3\"@ contains only the version
+-- For example the version range @\>= 3 && <= 3@ contains only the version
 -- @3@.
+--
+-- >>> isSpecificVersion (orLaterVersion (mkVersion [3]) `intersectVersionRanges` orEarlierVersion (mkVersion [3]))
+-- Just (mkVersion [3])
 isSpecificVersion :: VersionRange -> Maybe Version
 isSpecificVersion vr = case asVersionIntervals vr of
   [VersionInterval (LowerBound v InclusiveBound) (UpperBound v' InclusiveBound)]
@@ -222,5 +229,6 @@ transformCaretLower = hyloVersionRange embed projectVersionRange
 -- >>> :set -XScopedTypeVariables
 -- >>> import Distribution.Parsec
 -- >>> import Distribution.Pretty
+-- >>> import Distribution.Utils.Generic (ordNub)
 -- >>>
 -- >>> mapVR f xs = [pretty $ f v| Just v <- simpleParsec <$> xs]
diff --git a/cabal-install-solver/src/Distribution/Solver/Modular/Message.hs b/cabal-install-solver/src/Distribution/Solver/Modular/Message.hs
@@ -309,7 +309,7 @@ showFR _ (PackageRequiresMissingComponent qpn comp) = " (requires " ++ showExpos
 showFR _ (PackageRequiresPrivateComponent qpn comp) = " (requires " ++ showExposedComponent comp ++ " from " ++ showQPN qpn ++ ", but the component is private)"
 showFR _ (PackageRequiresUnbuildableComponent qpn comp) = " (requires " ++ showExposedComponent comp ++ " from " ++ showQPN qpn ++ ", but the component is not buildable in the current environment)"
 showFR _ CannotReinstall                  = " (avoiding to reinstall a package with same version but new dependencies)"
-showFR _ NotExplicit                      = " (not a user-provided goal nor mentioned as a constraint, but reject-unconstrained-dependencies was set)"
+showFR _ NotExplicit                      = " (not a user-provided goal nor mentioned as a constraint when reject-unconstrained-dependencies=all)"
 showFR _ Shadowed                         = " (shadowed by another installed package with same version)"
 showFR _ (Broken u)                       = " (package is broken, missing dependency " ++ prettyShow u ++ ")"
 showFR _ UnknownPackage                   = " (unknown package)"

diff --git a/cabal-install-solver/src/Distribution/Solver/Modular/Solver.hs b/cabal-install-solver/src/Distribution/Solver/Modular/Solver.hs
@@ -1,4 +1,5 @@
 {-# LANGUAGE CPP #-}
+{-# LANGUAGE ViewPatterns #-}
 #ifdef DEBUG_TRACETREE
 {-# OPTIONS_GHC -Wno-orphans #-}
 #endif
@@ -18,6 +19,7 @@ import Distribution.Verbosity
 
 import Distribution.Compiler (CompilerInfo)
 
+import Distribution.Version
 import Distribution.Solver.Types.PackagePath
 import Distribution.Solver.Types.PackagePreferences
 import Distribution.Solver.Types.PkgConfigDb (PkgConfigDb)
@@ -41,6 +43,7 @@ import Distribution.Solver.Modular.PSQ (PSQ)
 import Distribution.Solver.Modular.RetryLog
 import Distribution.Solver.Modular.Tree
 import qualified Distribution.Solver.Modular.PSQ as PSQ
+import Distribution.Solver.Types.PackageConstraint
 
 import Distribution.Simple.Setup (BooleanFlag(..))
 
@@ -140,15 +143,13 @@ solve sc cinfo idx pkgConfigDB userPrefs userConstraints userGoals =
     prunePhase       = (if asBool (avoidReinstalls sc) then P.avoidReinstalls (const True) else id) .
                        (case onlyConstrained sc of
                           OnlyConstrainedAll ->
-                            P.onlyConstrained pkgIsExplicit
+                            P.onlyConstrained (`S.member` versionConstrainedOrGoals)
                           OnlyConstrainedNone ->
                             id)
     buildPhase       = buildTree idx (independentGoals sc) (S.toList userGoals)
 
-    allExplicit = M.keysSet userConstraints `S.union` userGoals
-
-    pkgIsExplicit :: PN -> Bool
-    pkgIsExplicit pn = S.member pn allExplicit
+    versionConstrained = filterVersion isVersionConstrained userConstraints
+    versionConstrainedOrGoals = versionConstrained `S.union` userGoals
 
     -- When --reorder-goals is set, we use preferReallyEasyGoalChoices, which
     -- prefers (keeps) goals only if the have 0 or 1 enabled choice.
@@ -166,6 +167,25 @@ solve sc cinfo idx pkgConfigDB userPrefs userConstraints userGoals =
       | asBool (reorderGoals sc) = P.preferReallyEasyGoalChoices
       | otherwise                = id {- P.firstGoal -}
 
+-- | Keep package names of constraints that satisfy the predicate.
+filterVersion :: (LabeledPackageConstraint -> Bool) -> M.Map PN [LabeledPackageConstraint] -> Set PN
+filterVersion versionFilter = M.keysSet . M.filter (not . null) . M.map (filter versionFilter)
+
+normalise :: VersionRange -> VersionRange
+normalise = fromVersionIntervals . toVersionIntervals
+
+-- | Unconstrained with a version range @>=0@ or @<0@ or their flag equivalents
+-- and constrained by other versions ranges.
+--
+-- Both the @-any@ and @-none@ flags are considered unconstrained, because they
+-- don't actually constrain the version of the package. The @-any@ flag allows
+-- any version, and the @-none@ flag effectively excludes a package.
+isVersionConstrained :: LabeledPackageConstraint -> Bool
+isVersionConstrained (LabeledPackageConstraint (PackageConstraint _ c) _) = case c of
+    PackagePropertyVersion (normalise -> vr) -> not (isAnyVersion vr || isNoVersion vr)
+    -- `PackagePropertyFlags` @-any@ and @-none@ are covered below.
+    _ -> False
+
 -- | Dump solver tree to a file (in debugging mode)
 --
 -- This only does something if the @debug-tracetree@ configure argument was

diff --git a/cabal-install/tests/UnitTests/Distribution/Solver/Modular/Solver.hs b/cabal-install/tests/UnitTests/Distribution/Solver/Modular/Solver.hs
@@ -264,8 +264,8 @@ tests =
                   "Could not resolve dependencies:\n"
                     ++ "[__0] trying: A-3 (user goal)\n"
                     ++ "[__1] next goal: C (dependency of A)\n"
-                    ++ "[__1] fail (not a user-provided goal nor mentioned as a constraint, "
-                    ++ "but reject-unconstrained-dependencies was set)\n"
+                    ++ "[__1] fail (not a user-provided goal nor mentioned as a constraint"
+                    ++ " when reject-unconstrained-dependencies=all)\n"
                     ++ "[__1] fail (backjumping, conflict set: A, C)\n"
                     ++ "After searching the rest of the dependency tree exhaustively, "
                     ++ "these were the goals I've had most trouble fulfilling: A, C, B"

diff --git a/cabal-testsuite/PackageTests/RequireExplicit/FlagNotVersion/a.cabal b/cabal-testsuite/PackageTests/RequireExplicit/FlagNotVersion/a.cabal
@@ -0,0 +1,7 @@
+cabal-version: 2.2
+name: a
+version: 0
+
+library
+  build-depends:
+    some-lib
diff --git a/cabal-testsuite/PackageTests/RequireExplicit/FlagNotVersion/cabal.any-flag.out b/cabal-testsuite/PackageTests/RequireExplicit/FlagNotVersion/cabal.any-flag.out
@@ -0,0 +1,14 @@
+# tar
+# tar
+# tar
+# cabal v2-update
+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 when reject-unconstrained-dependencies=all)
+[__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)
diff --git a/cabal-testsuite/PackageTests/RequireExplicit/FlagNotVersion/cabal.any-version.out b/cabal-testsuite/PackageTests/RequireExplicit/FlagNotVersion/cabal.any-version.out
@@ -0,0 +1,14 @@
+# tar
+# tar
+# tar
+# cabal v2-update
+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 when reject-unconstrained-dependencies=all)
+[__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)
diff --git a/cabal-testsuite/PackageTests/RequireExplicit/FlagNotVersion/cabal.none-flag.out b/cabal-testsuite/PackageTests/RequireExplicit/FlagNotVersion/cabal.none-flag.out
@@ -0,0 +1,14 @@
+# tar
+# tar
+# tar
+# cabal v2-update
+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 when reject-unconstrained-dependencies=all)
+[__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)
diff --git a/cabal-testsuite/PackageTests/RequireExplicit/FlagNotVersion/cabal.none-version.out b/cabal-testsuite/PackageTests/RequireExplicit/FlagNotVersion/cabal.none-version.out
@@ -0,0 +1,14 @@
+# tar
+# tar
+# tar
+# cabal v2-update
+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 when reject-unconstrained-dependencies=all)
+[__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)
diff --git a/cabal-testsuite/PackageTests/RequireExplicit/FlagNotVersion/cabal.out b/cabal-testsuite/PackageTests/RequireExplicit/FlagNotVersion/cabal.out
@@ -0,0 +1,14 @@
+# tar
+# tar
+# tar
+# cabal v2-update
+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 when reject-unconstrained-dependencies=all)
+[__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)
diff --git a/cabal-testsuite/PackageTests/RequireExplicit/FlagNotVersion/cabal.project b/cabal-testsuite/PackageTests/RequireExplicit/FlagNotVersion/cabal.project
@@ -0,0 +1,3 @@
+packages: .
+constraints: some-lib +some-flag
+reject-unconstrained-dependencies: all
diff --git a/cabal-testsuite/PackageTests/RequireExplicit/FlagNotVersion/cabal.test.hs b/cabal-testsuite/PackageTests/RequireExplicit/FlagNotVersion/cabal.test.hs
@@ -0,0 +1,24 @@
+import Test.Cabal.Prelude
+main = do
+  cabalTest . recordMode RecordMarked $ withRepo "repo" $ do
+    res <- fails $ cabal' "v2-build" ["all", "--dry-run"]
+    assertOutputContains "not a user-provided goal" res
+
+  -- The following all check version ranges that don't have a version
+  -- constrained dependency one way or another.
+
+  cabalTest' "any-flag" . recordMode RecordMarked $ withRepo "repo" $ do
+    res <- fails $ cabal' "v2-build" ["all", "--dry-run", "--constraint", "some-lib -any"]
+    assertOutputContains "not a user-provided goal" res
+
+  cabalTest' "any-version" . recordMode RecordMarked $ withRepo "repo" $ do
+    res <- fails $ cabal' "v2-build" ["all", "--dry-run", "--constraint", "some-lib >=0"]
+    assertOutputContains "not a user-provided goal" res
+
+  cabalTest' "none-flag" . recordMode RecordMarked $ withRepo "repo" $ do
+    res <- fails $ cabal' "v2-build" ["all", "--dry-run", "--constraint", "some-lib -none"]
+    assertOutputContains "not a user-provided goal" res
+
+  cabalTest' "none-version" . recordMode RecordMarked $ withRepo "repo" $ do
+    res <- fails $ cabal' "v2-build" ["all", "--dry-run", "--constraint", "some-lib <0"]
+    assertOutputContains "not a user-provided goal" res
diff --git a/cabal-testsuite/PackageTests/RequireExplicit/FlagNotVersion/repo b/cabal-testsuite/PackageTests/RequireExplicit/FlagNotVersion/repo
@@ -0,0 +1 @@
+../repo/
diff --git a/changelog.d/12191.md b/changelog.d/12191.md
@@ -0,0 +1,11 @@
+---
+synopsis: Reject dependencies with unconstrained versions
+packages: [cabal-install]
+prs: 12191
+issues: 12190
+---
+
+Change the behaviour of `--reject-unconstrained-dependencies=all` so that it is
+not satisfied by flag constraints. Only version constraints should satisfy this
+check and even then these need to reject at least some versions so `>=0` or
+`-any` and `<0` or `-none` as version constraint will also fail the check.