[2.x] fix: Fixes subproject deps with different Scala versions (#8681)

When a project depended on another project that was built with a different Scala binary version (e.g. 2.12 vs 2.13), compilation could fail with "not found: value X" because resolution was asking for the wrong artifact.

This change updates how we build the `ModuleID` for inter-project dependencies in `projectDependenciesTask`: we now request the dependency’s Scala binary version (e.g. `bar_2.12`) instead of the current project’s, so the resolver can find the right artifact. We keep existing behavior for `Disabled` and `Constant` cross-version, and add a small safeguard in the default case when the dependency’s Scala version differs from the current project’s.

main/src/main/scala/sbt/Defaults.scala

@@ -4230,9 +4230,34 @@ object Classpaths {
                 .withCrossVersion(CrossVersion.constant(b.prefix + depSBV))
                 .withConfigurations(dep.configuration)
                 .withExplicitArtifacts(Vector.empty)
-            case _ if depAuto && VirtualAxis.isScala2Scala3Sandwich(sbv, depSBV) =>
+            case b: CrossVersion.Binary if sbv != depSBV =>
               depProjId
-                .withCrossVersion(CrossVersion.constant(depSBV))
+                .withCrossVersion(CrossVersion.constant(b.prefix + depSBV + b.suffix))
+                .withConfigurations(dep.configuration)
+                .withExplicitArtifacts(Vector.empty)
+            case f: CrossVersion.Full if sbv != depSBV =>
+              val cross = (dep.project / scalaVersion)
+                .get(data)
+                .map(sv => CrossVersion.constant(f.prefix + sv + f.suffix))
+                .getOrElse(depProjId.crossVersion)
+              depProjId
+                .withCrossVersion(cross)
+                .withConfigurations(dep.configuration)
+                .withExplicitArtifacts(Vector.empty)
+            // For3Use2_13/For2_13Use3 publish under compat suffix (e.g. bar_2.13 on Scala 3),
+            // not raw depSBV; sandwich case uses constant(depSBV) so would request wrong artifact.
+            case c: sbt.librarymanagement.For3Use2_13 if sbv != depSBV =>
+              val compat =
+                if (depSBV == "3" || depSBV.startsWith("3.0.0")) "2.13"
+                else depSBV
+              depProjId
+                .withCrossVersion(CrossVersion.constant(c.prefix + compat + c.suffix))
+                .withConfigurations(dep.configuration)
+                .withExplicitArtifacts(Vector.empty)
+            case c: sbt.librarymanagement.For2_13Use3 if sbv != depSBV =>
+              val compat = if (depSBV == "2.13") "3" else depSBV
+              depProjId
+                .withCrossVersion(CrossVersion.constant(c.prefix + compat + c.suffix))
                 .withConfigurations(dep.configuration)
                 .withExplicitArtifacts(Vector.empty)
             case _ =>

sbt-app/src/sbt-test/dependency-management/i4847-inter-project-variant-scala/bar/src/main/scala/Bar.scala

@@ -0,0 +1,3 @@
+object Bar {
+  def value: String = "from-bar-2.12"
+}

sbt-app/src/sbt-test/dependency-management/i4847-inter-project-variant-scala/baz/src/main/scala/Main.scala

@@ -0,0 +1,9 @@
+import java.nio.file.Files
+import java.nio.file.Paths
+
+object Main {
+  def main(args: Array[String]): Unit = {
+    val msg = Bar.value
+    Files.write(Paths.get("baz/output"), msg.getBytes("UTF-8"))
+  }
+}

sbt-app/src/sbt-test/dependency-management/i4847-inter-project-variant-scala/build.sbt

@@ -0,0 +1,19 @@
+// sbt#4847: inter-project deps with variant Scala binary versions.
+// bar uses 2.12, baz uses 2.13; baz dependsOn(bar). Resolution must ask for bar_2.12, not bar_2.13.
+ThisBuild / organization := "com.example"
+ThisBuild / version := "0.1.0-SNAPSHOT"
+
+lazy val foo = project in file(".")
+aggregateProjects(bar, baz)
+
+lazy val bar = project.settings(
+  scalaVersion := "2.12.18",
+  name := "bar",
+)
+
+lazy val baz = project
+  .dependsOn(bar)
+  .settings(
+    scalaVersion := "2.13.12",
+    name := "baz",
+  )

sbt-app/src/sbt-test/dependency-management/i4847-inter-project-variant-scala/test

@@ -0,0 +1,5 @@
+# sbt#4847: baz (2.13) dependsOn bar (2.12); update must resolve bar_2.12, then baz/run must succeed.
+$ delete baz/output
+> baz/update
+> baz/run
+$ exists baz/output