Make incremental compilation aware of synthesized mirrors

compiler/src/dotty/tools/dotc/sbt/ExtractAPI.scala

@@ -676,11 +676,16 @@ private class ExtractAPICollector(using Context) extends ThunkHolder {
 
     // In the Scala2 ExtractAPI phase we only extract annotations that extend
     // StaticAnnotation, but in Dotty we currently pickle all annotations so we
-    // extract everything (except annotations missing from the classpath which
-    // we simply skip over, and inline body annotations which are handled above).
+    // extract everything, except:
+    // - annotations missing from the classpath which we simply skip over
+    // - inline body annotations which are handled above
+    // - the Child annotation since we already extract children via
+    //   `api.ClassLike#childrenOfSealedClass` and adding this annotation would
+    //   lead to overcompilation when using zinc's
+    //   `IncOptions#useOptimizedSealed`.
     s.annotations.foreach { annot =>
       val sym = annot.symbol
-      if sym.exists && sym != defn.BodyAnnot then
+      if sym.exists && sym != defn.BodyAnnot && sym != defn.ChildAnnot then
         annots += apiAnnotation(annot)
     }
 

sbt-test/source-dependencies/useOptimizedSealed/Sealed.scala

@@ -0,0 +1,2 @@
+sealed trait Sealed
+class Child1 extends Sealed

sbt-test/source-dependencies/useOptimizedSealed/Test.scala

@@ -0,0 +1,3 @@
+class Test {
+  val s: Sealed = new Child1
+}

sbt-test/source-dependencies/useOptimizedSealed/build.sbt

@@ -0,0 +1,29 @@
+scalaVersion := sys.props("plugin.scalaVersion")
+
+ThisBuild / incOptions ~= { _.withUseOptimizedSealed(true) }
+
+import sbt.internal.inc.Analysis
+import complete.DefaultParsers._
+
+// Reset compiler iterations, necessary because tests run in batch mode
+val recordPreviousIterations = taskKey[Unit]("Record previous iterations.")
+recordPreviousIterations := {
+  val log = streams.value.log
+  CompileState.previousIterations = {
+    val previousAnalysis = (previousCompile in Compile).value.analysis.asScala
+    previousAnalysis match {
+      case None =>
+        log.info("No previous analysis detected")
+        0
+      case Some(a: Analysis) => a.compilations.allCompilations.size
+    }
+  }
+}
+
+val checkIterations = inputKey[Unit]("Verifies the accumulated number of iterations of incremental compilation.")
+
+checkIterations := {
+  val expected: Int = (Space ~> NatBasic).parsed
+  val actual: Int = ((compile in Compile).value match { case a: Analysis => a.compilations.allCompilations.size }) - CompileState.previousIterations
+  assert(expected == actual, s"Expected $expected compilations, got $actual")
+}

sbt-test/source-dependencies/useOptimizedSealed/changes/Sealed1.scala

@@ -0,0 +1,3 @@
+sealed trait Sealed
+class Child1 extends Sealed
+class Child2 extends Sealed

sbt-test/source-dependencies/useOptimizedSealed/changes/Sealed2.scala

@@ -0,0 +1,4 @@
+sealed trait Sealed
+class Child1 extends Sealed
+class Child2 extends Sealed
+class Child3 extends Sealed

sbt-test/source-dependencies/useOptimizedSealed/changes/Test1.scala

@@ -0,0 +1,7 @@
+class Test {
+  def foo(x: Sealed): Int = x match
+    case _: Child1 => 1
+    case _: Child2 => 1
+
+  val s: Sealed = new Child1
+}

sbt-test/source-dependencies/useOptimizedSealed/project/CompileState.scala

@@ -0,0 +1,4 @@
+// This is necessary because tests are run in batch mode
+object CompileState {
+  @volatile var previousIterations: Int = -1
+}

sbt-test/source-dependencies/useOptimizedSealed/test

@@ -0,0 +1,24 @@
+# Compile Sealed.scala and Test.scala
+> compile
+> recordPreviousIterations
+
+# Add an extra children to Sealed
+$ copy-file changes/Sealed1.scala Sealed.scala
+
+# Only Sealed.scala needs to be recompiled because Test.scala does not
+# match on a value of type `Sealed`.
+> compile
+> checkIterations 1
+
+> clean
+$ copy-file changes/Test1.scala Test.scala
+> compile
+> recordPreviousIterations
+
+# Add an extra children to Sealed again
+$ copy-file changes/Sealed2.scala Sealed.scala
+
+# Test.scala will be recompiled in a second round because it matches
+# on a value of type `Sealed`.
+> compile
+> checkIterations 2