Allow to force some module versions during resolution

What SBT calls "dependency overrides"

core/shared/src/main/scala/coursier/core/Resolution.scala

@@ -163,25 +163,34 @@ object Resolution {
    * Returns the conflicted dependencies, and the merged others.
    */
   def merge(
-    dependencies: TraversableOnce[Dependency]
+    dependencies: TraversableOnce[Dependency],
+    forceVersions: Map[Module, String]
   ): (Seq[Dependency], Seq[Dependency]) = {
 
     val mergedByModVer = dependencies
       .toList
       .groupBy(dep => dep.module)
-      .mapValues { deps =>
+      .map { case (module, deps) =>
-        if (deps.lengthCompare(1) == 0) \/-(deps)
+        module -> {
-        else {
+          forceVersions.get(module) match {
-          val versions = deps
-            .map(_.version)
-            .distinct
-          val versionOpt = mergeVersions(versions)
-
-          versionOpt match {
-            case Some(version) =>
-              \/-(deps.map(dep => dep.copy(version = version)))
             case None =>
-              -\/(deps)
+              if (deps.lengthCompare(1) == 0) \/-(deps)
+              else {
+                val versions = deps
+                  .map(_.version)
+                  .distinct
+                val versionOpt = mergeVersions(versions)
+
+                versionOpt match {
+                  case Some(version) =>
+                    \/-(deps.map(dep => dep.copy(version = version)))
+                  case None =>
+                    -\/(deps)
+                }
+              }
+
+            case Some(forcedVersion) =>
+              \/-(deps.map(dep => dep.copy(version = forcedVersion)))
           }
         }
       }
@@ -380,6 +389,7 @@ object Resolution {
 case class Resolution(
   rootDependencies: Set[Dependency],
   dependencies: Set[Dependency],
+  forceVersions: Map[Module, String],
   conflicts: Set[Dependency],
   projectCache: Map[Resolution.ModuleVersion, (Artifact.Source, Project)],
   errorCache: Map[Resolution.ModuleVersion, Seq[String]],
@@ -426,9 +436,10 @@ case class Resolution(
    * the dependencies.
    */
   def nextDependenciesAndConflicts: (Seq[Dependency], Seq[Dependency]) =
+    // TODO Provide the modules whose version was forced by dependency overrides too
     merge(
-      rootDependencies.map(withDefaultScope) ++ dependencies ++
+      rootDependencies.map(withDefaultScope) ++ dependencies ++ transitiveDependencies,
-      transitiveDependencies
+      forceVersions
     )
 
   /**

core/shared/src/main/scala/coursier/package.scala

@@ -63,6 +63,7 @@ package object coursier {
     def apply(
       rootDependencies: Set[Dependency] = Set.empty,
       dependencies: Set[Dependency] = Set.empty,
+      forceVersions: Map[Module, String] = Map.empty,
       conflicts: Set[Dependency] = Set.empty,
       projectCache: Map[ModuleVersion, (Artifact.Source, Project)] = Map.empty,
       errorCache: Map[ModuleVersion, Seq[String]] = Map.empty,
@@ -72,6 +73,7 @@ package object coursier {
       core.Resolution(
         rootDependencies,
         dependencies,
+        forceVersions,
         conflicts,
         projectCache,
         errorCache,

tests/shared/src/test/scala/coursier/test/ResolutionTests.scala

@@ -9,12 +9,15 @@ import coursier.test.compatibility._
 
 object ResolutionTests extends TestSuite {
 
-  def resolve0(deps: Set[Dependency], filter: Option[Dependency => Boolean] = None) = {
+  def resolve0(
-    Resolution(deps, filter = filter)
+    deps: Set[Dependency],
+    filter: Option[Dependency => Boolean] = None,
+    forceVersions: Map[Module, String] = Map.empty
+  ) =
+    Resolution(deps, filter = filter, forceVersions = forceVersions)
       .process
       .run(Fetch.default(repositories))
       .runF
-  }
 
   implicit class ProjectOps(val p: Project) extends AnyVal {
     def kv: (ModuleVersion, (Artifact.Source, Project)) = p.moduleVersion -> (testRepository.source, p)
@@ -134,9 +137,16 @@ object ResolutionTests extends TestSuite {
 
     Project(Module("an-org", "a-name"), "1.0"),
 
+    Project(Module("an-org", "a-name"), "1.2"),
+
     Project(Module("an-org", "a-lib"), "1.0",
       Seq(Dependency(Module("an-org", "a-name"), "1.0"))),
 
+    Project(Module("an-org", "a-lib"), "1.1"),
+
+    Project(Module("an-org", "a-lib"), "1.2",
+      Seq(Dependency(Module("an-org", "a-name"), "1.2"))),
+
     Project(Module("an-org", "another-lib"), "1.0",
       Seq(Dependency(Module("an-org", "a-name"), "1.0"))),
 
@@ -144,7 +154,15 @@ object ResolutionTests extends TestSuite {
     Project(Module("an-org", "an-app"), "1.0",
       Seq(
         Dependency(Module("an-org", "a-lib"), "1.0", exclusions = Set(("an-org", "a-name"))),
-        Dependency(Module("an-org", "another-lib"), "1.0", optional = true)))
+        Dependency(Module("an-org", "another-lib"), "1.0", optional = true))),
+
+    Project(Module("an-org", "an-app"), "1.1",
+      Seq(
+        Dependency(Module("an-org", "a-lib"), "1.1"))),
+
+    Project(Module("an-org", "an-app"), "1.2",
+      Seq(
+        Dependency(Module("an-org", "a-lib"), "1.2")))
   )
 
   val projectsMap = projects.map(p => p.moduleVersion -> p).toMap
@@ -483,6 +501,86 @@ object ResolutionTests extends TestSuite {
       }
     }
 
+    'dependencyOverrides - {
+      * - {
+        async {
+          val deps = Set(
+            Dependency(Module("an-org", "a-name"), "1.1"))
+          val depOverrides = Map(
+            Module("an-org", "a-name") -> "1.0")
+
+          val res = await(resolve0(
+            deps,
+            forceVersions = depOverrides,
+            filter = Some(_.scope == Scope.Compile)
+          )).copy(filter = None, projectCache = Map.empty, errorCache = Map.empty)
+
+          val expected = Resolution(
+            rootDependencies = deps,
+            dependencies = Set(
+              Dependency(Module("an-org", "a-name"), "1.0")
+            ).map(_.withCompileScope),
+            forceVersions = depOverrides
+          )
+
+          assert(res == expected)
+        }
+      }
+
+      * - {
+        async {
+          val deps = Set(
+            Dependency(Module("an-org", "an-app"), "1.1"))
+          val depOverrides = Map(
+            Module("an-org", "a-lib") -> "1.0")
+
+          val res = await(resolve0(
+            deps,
+            forceVersions = depOverrides,
+            filter = Some(_.scope == Scope.Compile)
+          )).copy(filter = None, projectCache = Map.empty, errorCache = Map.empty)
+
+          val expected = Resolution(
+            rootDependencies = deps,
+            dependencies = Set(
+              Dependency(Module("an-org", "an-app"), "1.1"),
+              Dependency(Module("an-org", "a-lib"), "1.0"),
+              Dependency(Module("an-org", "a-name"), "1.0")
+            ).map(_.withCompileScope),
+            forceVersions = depOverrides
+          )
+
+          assert(res == expected)
+        }
+      }
+
+      * - {
+        async {
+          val deps = Set(
+            Dependency(Module("an-org", "an-app"), "1.2"))
+          val depOverrides = Map(
+            Module("an-org", "a-lib") -> "1.1")
+
+          val res = await(resolve0(
+            deps,
+            forceVersions = depOverrides,
+            filter = Some(_.scope == Scope.Compile)
+          )).copy(filter = None, projectCache = Map.empty, errorCache = Map.empty)
+
+          val expected = Resolution(
+            rootDependencies = deps,
+            dependencies = Set(
+              Dependency(Module("an-org", "an-app"), "1.2"),
+              Dependency(Module("an-org", "a-lib"), "1.1")
+            ).map(_.withCompileScope),
+            forceVersions = depOverrides
+          )
+
+          assert(res == expected)
+        }
+      }
+    }
+
     'parts{
       'propertySubstitution{
         val res =