Add solver unit tests for sub-library dependencies (issue haskell#6039).

cabal-install/tests/UnitTests/Distribution/Solver/Modular/Solver.hs

@@ -24,6 +24,7 @@ import Distribution.Solver.Types.PackageConstraint
 import qualified Distribution.Solver.Types.PackagePath as P
 import UnitTests.Distribution.Solver.Modular.DSL
 import UnitTests.Distribution.Solver.Modular.DSL.TestCaseUtils
+import Distribution.Types.LibraryVisibility
 
 tests :: [TF.TestTree]
 tests = [
@@ -281,7 +282,7 @@ tests = [
         , runTest $         mkTest dbBJ7  "bj7"  ["A"]      (solverSuccess [("A", 1), ("B",  1), ("C", 1)])
         , runTest $ indep $ mkTest dbBJ8  "bj8"  ["A", "B"] (solverSuccess [("A", 1), ("B",  1), ("C", 1)])
         ]
-    , testGroup "library dependencies" [
+    , testGroup "main library dependencies" [
           let db = [Right $ exAvNoLibrary "A" 1 `withExe` ExExe "exe" []]
           in runTest $ mkTest db "install build target without a library" ["A"] $
              solverSuccess [("A", 1)]
@@ -298,6 +299,46 @@ tests = [
           in runTest $ mkTest db "choose version of build-depends dependency that has a library" ["A"] $
              solverSuccess [("A", 1), ("B", 1)]
         ]
+    , testGroup "sub-library dependencies" [
+          let db = [ Right $ exAv "A" 1 [ExSubLibAny "B" "sub-lib"]
+                   , Right $ exAv "B" 1 [] ]
+          in runTest $
+             mkTest db "reject package that is missing required sub-library" ["A"] $
+             solverFailure $ isInfixOf $
+             "rejecting: B-1.0.0 (does not contain library 'sub-lib', which is required by A)"
+
+        , let db = [ Right $ exAv "A" 1 [ExSubLibAny "B" "sub-lib"]
+                   , Right $ exAv "B" 1 [] `withSubLibrary` ExSubLib "sub-lib" LibraryVisibilityPrivate [] ]
+          in runTest $
+             mkTest db "reject package with private but required sub-library" ["A"] $
+             solverFailure $ isInfixOf $ "rejecting: B-1.0.0 (library 'sub-lib' is private, but it is required by A)"
+
+        , let db = [ Right $ exAv "A" 1 [ExAny "B"]
+                   , Right $ exAv "B" 1 [] `withSubLibrary` ExSubLib "sub-lib" LibraryVisibilityPrivate []
+                   , Right $ exAv "C" 1 [ExSubLibAny "B" "sub-lib"] ]
+              goals :: [ExampleVar]
+              goals = [
+                  P QualNone "A"
+                , P QualNone "B"
+                , P QualNone "C"
+                ]
+          in runTest $ goalOrder goals $
+             mkTest db "reject package that requires a private sub-library" ["A", "C"] $
+             solverFailure $ isInfixOf $
+             "rejecting: C-1.0.0 (requires library 'sub-lib' from B, but the component is private)"
+
+        , let db = [ Right $ exAv "A" 1 [ExSubLibAny "B" "sub-lib-v1"]
+                   , Right $ exAv "B" 2 [] `withSubLibrary` ExSubLib "sub-lib-v2" LibraryVisibilityPublic []
+                   , Right $ exAv "B" 1 [] `withSubLibrary` ExSubLib "sub-lib-v1" LibraryVisibilityPublic [] ]
+          in runTest $ mkTest db "choose version of package containing correct sub-library" ["A"] $
+             solverSuccess [("A", 1), ("B", 1)]
+
+        , let db = [ Right $ exAv "A" 1 [ExSubLibAny "B" "sub-lib"]
+                   , Right $ exAv "B" 2 [] `withSubLibrary` ExSubLib "sub-lib" LibraryVisibilityPrivate []
+                   , Right $ exAv "B" 1 [] `withSubLibrary` ExSubLib "sub-lib" LibraryVisibilityPublic [] ]
+          in runTest $ mkTest db "choose version of package with public sub-library" ["A"] $
+             solverSuccess [("A", 1), ("B", 1)]
+        ]
     -- build-tool-depends dependencies
     , testGroup "build-tool-depends" [
           runTest $ mkTest dbBuildTools "simple exe dependency" ["A"] (solverSuccess [("A", 1), ("bt-pkg", 2)])