From 728e2d36fdae6d48d00cd6b9f4ee175295dc5fc5 Mon Sep 17 00:00:00 2001
From: Piotr Chabelski <pchabelski@virtuslab.com>
Date: Mon, 19 Sep 2022 14:58:06 +0200
Subject: [PATCH 1/2] Fix how Scala version is picked so that `2.12.1` is
 supported and respected

---
 .../scala/build/options/ScalaVersionUtil.scala    | 15 ++++++++-------
 1 file changed, 8 insertions(+), 7 deletions(-)

diff --git a/modules/options/src/main/scala/scala/build/options/ScalaVersionUtil.scala b/modules/options/src/main/scala/scala/build/options/ScalaVersionUtil.scala
index c25482884e..378167b0cb 100644
--- a/modules/options/src/main/scala/scala/build/options/ScalaVersionUtil.scala
+++ b/modules/options/src/main/scala/scala/build/options/ScalaVersionUtil.scala
@@ -221,13 +221,14 @@ object ScalaVersionUtil {
         if (filtered.isEmpty) matchingStableVersions
         else filtered
       }.filter(v => isSupportedVersion(v.repr))
-      if (validMatchingVersions.isEmpty)
-        Left(new UnsupportedScalaVersionError(
-          scalaVersionStringArg,
-          latestSupportedStableVersions
-        ))
-      else
-        Right(validMatchingVersions.max.repr)
+
+      validMatchingVersions.find(_.repr == scalaVersionStringArg) match {
+        case Some(v)                                => Right(v.repr)
+        case None if validMatchingVersions.nonEmpty => Right(validMatchingVersions.max.repr)
+        case _ => Left(
+            new UnsupportedScalaVersionError(scalaVersionStringArg, latestSupportedStableVersions)
+          )
+      }
     }
   }
 

From 59ab39b9ea374fb032a0f6a2d4dad787019ddf6f Mon Sep 17 00:00:00 2001
From: Piotr Chabelski <pchabelski@virtuslab.com>
Date: Mon, 19 Sep 2022 14:59:32 +0200
Subject: [PATCH 2/2] Turn off unsupported `-release` compiler flag on Scala
 `2.12.x` < `2.12.5`

---
 .../build/src/main/scala/scala/build/Build.scala  | 15 +++++++++++----
 .../cli/integration/RunTestDefinitions.scala      | 14 ++++++++++++++
 .../scala/build/options/ScalaVersionUtil.scala    |  7 +++++++
 3 files changed, 32 insertions(+), 4 deletions(-)

diff --git a/modules/build/src/main/scala/scala/build/Build.scala b/modules/build/src/main/scala/scala/build/Build.scala
index 06ffe02d51..c7ea1237a2 100644
--- a/modules/build/src/main/scala/scala/build/Build.scala
+++ b/modules/build/src/main/scala/scala/build/Build.scala
@@ -18,13 +18,14 @@ import scala.build.compiler.{ScalaCompiler, ScalaCompilerMaker}
 import scala.build.errors.*
 import scala.build.internal.resource.ResourceMapper
 import scala.build.internal.{Constants, CustomCodeWrapper, MainClass, Util}
+import scala.build.options.ScalaVersionUtil.maybeScalaPatchVersion
 import scala.build.options.*
 import scala.build.options.validation.ValidationException
 import scala.build.postprocessing.*
 import scala.collection.mutable.ListBuffer
 import scala.concurrent.duration.DurationInt
-import scala.util.Properties
 import scala.util.control.NonFatal
+import scala.util.{Properties, Try}
 
 trait Build {
   def inputs: Inputs
@@ -825,9 +826,15 @@ object Build {
             Seq(ScalacOpt("-scalajs"))
           else Nil
 
-        val scalacReleaseV = releaseFlagVersion
-          .map(v => List("-release", v).map(ScalacOpt(_)))
-          .getOrElse(Nil)
+        val scalacReleaseV =
+          // the -release flag is not supported for Scala 2.12.x < 2.12.5
+          if params.scalaVersion.startsWith("2.12") &&
+            params.scalaVersion.maybeScalaPatchVersion.exists(_ < 5)
+          then Nil
+          else
+            releaseFlagVersion
+              .map(v => List("-release", v).map(ScalacOpt(_)))
+              .getOrElse(Nil)
 
         val scalapyOptions =
           if (
diff --git a/modules/integration/src/test/scala/scala/cli/integration/RunTestDefinitions.scala b/modules/integration/src/test/scala/scala/cli/integration/RunTestDefinitions.scala
index be10eeea68..59c34882be 100644
--- a/modules/integration/src/test/scala/scala/cli/integration/RunTestDefinitions.scala
+++ b/modules/integration/src/test/scala/scala/cli/integration/RunTestDefinitions.scala
@@ -2380,6 +2380,20 @@ abstract class RunTestDefinitions(val scalaVersionOpt: Option[String])
     }
   }
 
+  if (actualScalaVersion.startsWith("2.12."))
+    test("verify that Scala version 2.12.x < 2.12.4 is respected and compiles correctly") {
+      TestInputs(os.rel / "s.sc" -> "println(util.Properties.versionNumberString)").fromRoot {
+        root =>
+          (1 until 4).foreach { scalaPatchVersion =>
+            val scala212VersionString = s"2.12.$scalaPatchVersion"
+            val res =
+              os.proc(TestUtil.cli, "run", ".", "-S", scala212VersionString, TestUtil.extraOptions)
+                .call(cwd = root)
+            expect(res.out.trim == scala212VersionString)
+          }
+      }
+    }
+
   def scalapyNativeTest(): Unit = {
     val inputs = TestInputs(
       os.rel / "helloscalapy.sc" ->
diff --git a/modules/options/src/main/scala/scala/build/options/ScalaVersionUtil.scala b/modules/options/src/main/scala/scala/build/options/ScalaVersionUtil.scala
index 378167b0cb..3a9908b7b9 100644
--- a/modules/options/src/main/scala/scala/build/options/ScalaVersionUtil.scala
+++ b/modules/options/src/main/scala/scala/build/options/ScalaVersionUtil.scala
@@ -19,6 +19,7 @@ import scala.build.errors.{
 import scala.build.internal.Regexes.scala2NightlyRegex
 import scala.build.internal.Util
 import scala.concurrent.duration.DurationInt
+import scala.util.Try
 import scala.util.control.NonFatal
 
 object ScalaVersionUtil {
@@ -291,4 +292,10 @@ object ScalaVersionUtil {
       .distinct
   }
 
+  extension (sv: String) {
+    def maybeScalaPatchVersion: Option[Int] = sv
+      .split('.').drop(2).headOption
+      .flatMap(_.split('-').headOption)
+      .flatMap(pv => Try(pv.toInt).toOption)
+  }
 }