fix: Fix Scala 3.x - 2.12 sandwich for matrix

Problem
Currently Scala 3.x - 2.13 sandwich doesn't seem to work for projectMatrix.
This is due to isScala2Scala3Sandwich checking for "3." while projectMatrix
uses scalaBinaryVersion, which is 3.

Solution
This consolidates the implementation of isScala2Scala3Sandwich with the one
in Defaults module, and check for both "3" and "3."

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

@@ -4116,35 +4116,34 @@ object Classpaths {
 
   private[sbt] def isScala213(sv: String) = sv.startsWith("2.13.")
 
-  private[sbt] def isScala2Scala3Sandwich(sbv1: String, sbv2: String): Boolean = {
-    def compare(a: String, b: String): Boolean =
-      a == "2.13" && (b.startsWith("0.") || b.startsWith("3"))
-    compare(sbv1, sbv2) || compare(sbv2, sbv1)
-  }
-
   def projectDependenciesTask: Initialize[Task[Seq[ModuleID]]] =
     Def.task {
       val sbv = scalaBinaryVersion.value
       val ref = thisProjectRef.value
       val data = settingsData.value
       val deps = buildDependencies.value
-      deps.classpath(ref) flatMap { dep =>
-        for {
-          depProjId <- (dep.project / projectID).get(data)
-          depSBV <- (dep.project / scalaBinaryVersion).get(data)
-          depCross <- (dep.project / crossVersion).get(data)
-        } yield {
-          depCross match {
-            case b: CrossVersion.Binary if isScala2Scala3Sandwich(sbv, depSBV) =>
+      deps
+        .classpath(ref)
+        .flatMap: dep =>
+          for
+            depProjId <- (dep.project / projectID).get(data)
+            depSBV <- (dep.project / scalaBinaryVersion).get(data)
+            depCross <- (dep.project / crossVersion).get(data)
+            depAuto <- (dep.project / autoScalaLibrary).get(data)
+          yield depCross match
+            case b: CrossVersion.Binary
+                if depAuto && VirtualAxis.isScala2Scala3Sandwich(sbv, depSBV) =>
               depProjId
                 .withCrossVersion(CrossVersion.constant(b.prefix + depSBV))
                 .withConfigurations(dep.configuration)
                 .withExplicitArtifacts(Vector.empty)
+            case _ if depAuto && VirtualAxis.isScala2Scala3Sandwich(sbv, depSBV) =>
+              depProjId
+                .withCrossVersion(CrossVersion.constant(depSBV))
+                .withConfigurations(dep.configuration)
+                .withExplicitArtifacts(Vector.empty)
             case _ =>
               depProjId.withConfigurations(dep.configuration).withExplicitArtifacts(Vector.empty)
-          }
-        }
-      }
     }
 
   private[sbt] def depMap: Initialize[Task[Map[ModuleRevisionId, ModuleDescriptor]]] =

main/src/main/scala/sbt/ProjectMatrix.scala

@@ -333,7 +333,6 @@ object ProjectMatrix {
       name := self.id,
       sourceDirectory := base.getAbsoluteFile / "src",
       unmanagedBase := base.getAbsoluteFile / "lib",
-      projectDependencies := projectDependenciesTask.value,
       projectMatrixBaseDirectory := base,
     )
 

main/src/main/scala/sbt/VirtualAxis.scala

@@ -66,11 +66,10 @@ object VirtualAxis {
     isScala2Scala3Sandwich(str(sbv1), str(sbv2))
   }
 
-  private[sbt] def isScala2Scala3Sandwich(sbv1: String, sbv2: String): Boolean = {
+  private[sbt] def isScala2Scala3Sandwich(sbv1: String, sbv2: String): Boolean =
     def compare(a: String, b: String): Boolean =
-      a == "2.13" && (b.startsWith("0.") || b.startsWith("3."))
+      a == "2.13" && (b == "3" || b.startsWith("3."))
     compare(sbv1, sbv2) || compare(sbv2, sbv1)
-  }
 
   // This admits partial Scala version
   private[sbt] def isPartialVersionEquals(ax1: VirtualAxis, ax2: VirtualAxis): Boolean = {

sbt-app/src/sbt-test/project-matrix/jvm-sandwich/build.sbt

@@ -1,14 +1,20 @@
 lazy val check = taskKey[Unit]("")
-lazy val scala3M1 = "3.0.0-M1"
+lazy val scala3 = "3.5.2"
 lazy val scala3M2 = "3.0.0-M2"
-lazy val scala213 = "2.13.4"
+lazy val scala213 = "2.13.14"
 
 lazy val fooApp = (projectMatrix in file("foo-app"))
   .dependsOn(fooCore)
   .settings(
     name := "foo app",
+    TaskKey[Unit]("check") := {
+      val deps = (Compile / projectDependencies).value
+      deps.foreach { dep =>
+        println(s"$dep - ${dep.crossVersion}")
+      }
+    },
   )
-  .jvmPlatform(scalaVersions = Seq(scala3M1, scala3M2))
+  .jvmPlatform(scalaVersions = Seq(scala3, scala3M2))
 
 lazy val fooCore = (projectMatrix in file("foo-core"))
   .settings(
@@ -27,7 +33,7 @@ lazy val barCore = (projectMatrix in file("bar-core"))
   .settings(
     name := "bar core",
   )
-  .jvmPlatform(scalaVersions = Seq(scala3M1))
+  .jvmPlatform(scalaVersions = Seq(scala3))
 
 // choose 2.13 when bazCore offers both 2.13 and Dotty
 lazy val bazApp = (projectMatrix in file("baz-app"))
@@ -48,4 +54,4 @@ lazy val bazCore = (projectMatrix in file("baz-core"))
     name := "baz core",
     exportJars := true,
   )
-  .jvmPlatform(scalaVersions = Seq(scala213, scala3M1, scala3M2))
+  .jvmPlatform(scalaVersions = Seq(scala213, scala3, scala3M2))

sbt-app/src/sbt-test/project-matrix/jvm-sandwich/test

@@ -1,6 +1,8 @@
 > projects
 
-> fooApp3_0_0_M1/compile
+> fooApp/check
+
+> fooApp/compile
 
 > barApp2_13/compile