[build] Refine checkReleasedTastyVersion logic to handle using unstable reference version of a new minor (#22789)

In #22786 we've followed the process and updated referenceVersion to
latest released version which happens to be 3.7.0-RC1 producing TASTY
`28.7-experimental-1`.
All nightly version when patch > 0 should be using TASTy version
incremented by 1, in case of unexpected binary breakage. However
non-bootstrapped tests (eg. FromTastyTest) would fail if we'd increase
tasty version to `28.8-experimental-1` because these require the
produced and consumed Scala files (eg. in `FromTastyTest`) to either:
- be produced by older stable compiler 
- produced with exactly the same TASTy version 

Because of that we're adding special case to checking version of
released tasty to allow using the same tasty format as unstable
reference version if the reference version is the RC release of new
minor (patch == 0)

Fixes #22788

[skip ci]

Author: WojciechMazur

Diff for: project/Build.scala

@@ -2534,11 +2534,14 @@ object Build {
 
   /* Tests TASTy version invariants during NIGHLY, RC or Stable releases */
   def checkReleasedTastyVersion(): Unit = {
-    lazy val (scalaMinor, scalaPatch, scalaIsRC) = baseVersion.split("\\.|-").take(4) match {
-      case Array("3", minor, patch)    => (minor.toInt, patch.toInt, false)
-      case Array("3", minor, patch, _) => (minor.toInt, patch.toInt, true)
+    case class ScalaVersion(minor: Int, patch: Int, isRC: Boolean)
+    def parseScalaVersion(version: String): ScalaVersion = version.split("\\.|-").take(4) match {
+      case Array("3", minor, patch)    => ScalaVersion(minor.toInt, patch.toInt, false)
+      case Array("3", minor, patch, _) => ScalaVersion(minor.toInt, patch.toInt, true)
       case other => sys.error(s"Invalid Scala base version string: $baseVersion")
     }
+    lazy val version = parseScalaVersion(baseVersion)
+    lazy val referenceV = parseScalaVersion(referenceVersion)
     lazy val (tastyMinor, tastyIsExperimental) = expectedTastyVersion.split("\\.|-").take(4) match {
       case Array("28", minor)                    => (minor.toInt, false)
       case Array("28", minor, "experimental", _) => (minor.toInt, true)
@@ -2547,13 +2550,22 @@ object Build {
 
     if(isNightly) {
       assert(tastyIsExperimental, "TASTY needs to be experimental in nightly builds")
-      val expectedTastyMinor = if(scalaPatch == 0) scalaMinor else scalaMinor + 1
+      val expectedTastyMinor = version.patch match {
+        case 0 => version.minor
+        case 1 if referenceV.patch == 0 && referenceV.isRC =>
+          // Special case for a period when reference version is a new unstable minor
+          // Needed for non_bootstrapped tests requiring either stable tasty or the same experimental version produced by both reference and bootstrapped compiler
+          assert(version.minor == referenceV.minor, "Expected reference and base version to use the same minor")
+          version.minor
+        case _ => version.minor + 1
+      }
       assert(tastyMinor == expectedTastyMinor, "Invalid TASTy minor version")
     }
 
     if(isRelease) {
-      assert(scalaMinor == tastyMinor, "Minor versions of TASTY vesion and Scala version should match in release builds")
-      if (scalaIsRC && scalaPatch == 0)
+      assert(version.minor == tastyMinor, "Minor versions of TASTY vesion and Scala version should match in release builds")
+      assert(!referenceV.isRC, "Stable release needs to use stable compiler version")
+      if (version.isRC && version.patch == 0)
         assert(tastyIsExperimental, "TASTy should be experimental when releasing a new minor version RC")
       else
         assert(!tastyIsExperimental, "Stable version cannot use experimental TASTY")