Remove compatibility check from ++

Problem
-------
Since sbt-doge merger `++ <sv> <command1>` has used binary compatibility
as a test to select subproject, but it causes surprising situations like
sbt/sbt#6915, and it blurs the responsibility of YAML file and build
file as the version specified in the version can override the Scala
version test on local laptop.

Solution
--------
This removes the compatibiliy check (backward-only or otherwise),
and require that `<sv>` match one of `crossScalaVersions` using the new
Semantic Version selector pattern.

main/src/main/scala/sbt/Cross.scala

@@ -19,7 +19,7 @@ import sbt.internal.util.MessageOnlyException
 import sbt.internal.util.complete.DefaultParsers._
 import sbt.internal.util.complete.{ DefaultParsers, Parser }
 import sbt.io.IO
-import sbt.librarymanagement.{ CrossVersion, SemanticSelector, VersionNumber }
+import sbt.librarymanagement.{ SemanticSelector, VersionNumber }
 
 /**
  * Cross implements the Scala cross building commands:
@@ -334,10 +334,10 @@ object Cross {
         } ++ structure.units.keys
           .map(BuildRef.apply)
           .map(proj => (proj, Some(version), crossVersions(extracted, proj)))
-      } else if (isSelector(version)) {
-        val selector = SemanticSelector(version)
+      } else {
         projectScalaVersions.map {
           case (project, scalaVersions) =>
+            val selector = SemanticSelector(version)
             scalaVersions.filter(v => selector.matches(VersionNumber(v))) match {
               case Nil          => (project, None, scalaVersions)
               case Seq(version) => (project, Some(version), scalaVersions)
@@ -347,18 +347,7 @@ object Cross {
                 )
             }
         }
-      } else
-        // This is the default implementation for ++ <sv> <command1>
-        // It checks that <sv> is backward compatible with one of the Scala versions listed
-        // in crossScalaVersions setting. Note this must account for the fact that in Scala 3.x
-        // 3.1.0 is not compatible with 3.0.0.
-        projectScalaVersions.map {
-          case (project, scalaVersions) =>
-            if (scalaVersions.exists(CrossVersion.isScalaBinaryCompatibleWith(version, _)))
-              (project, Some(version), scalaVersions)
-            else
-              (project, None, scalaVersions)
-        }
+      }
     }
 
     val included = projects.collect {

main/src/main/scala/sbt/internal/CommandStrings.scala

@@ -387,17 +387,23 @@ $SwitchCommand [<scala-version>=]<scala-home>[!] [-v] [<command>]
 	Uses the Scala installation at <scala-home> by configuring the scalaHome setting for
 	all projects.
 
-	If <scala-version> is specified, it is used as the value of the scalaVersion setting.
+	If <scala-version> is specified, it is used to select the value of the scalaVersion setting
+  from one of the values of crossScalaVersions setting.
 	This is important when using managed dependencies.  This version will determine the
 	cross-version used as well as transitive dependencies.
 
-  Only projects that are listed to be binary compatible with the selected Scala version
+  <scala-version> may be an actual Scala version such as 3.1.3, or a Semantic Version selector
+  pattern such as 2.13.x. Only subprojects that are listed to match the version pattern
   have their Scala version switched.  If ! is supplied, then all projects projects have
   their Scala version switched.
 
   If -v is supplied, verbose logging of the Scala version switching is done.
 
-	If <command> is provided, it is then executed.
+	If <command> is provided, it is then executed. For example:
+
+    ++ 2.13.x test
+
+  The above will run test on all subprojects that contain a 2.13 Scala version.
 
 	See also `help $CrossCommand`
 """

sbt-app/src/sbt-test/actions/cross-strict-aggregation/build.sbt

@@ -1,4 +1,4 @@
-lazy val scala212 = "2.12.12"
+lazy val scala212 = "2.12.16"
 lazy val scala213 = "2.13.1"
 
 ThisBuild / scalaVersion := scala212

sbt-app/src/sbt-test/actions/cross-strict-aggregation/test

@@ -1,5 +1,7 @@
 -> ++2.12.0-magic
 
+-> ++2.12.12
+
 > ++2.13.1 compile
 
 $ exists core/target/scala-2.13