Fix under-compilation when the method type in a SAM changes

A lambda that implements a SAM type desugars (either at compile-time or runtime
via invokedynamic) into a class that extends the SAM type, therefore we need to
register an inheritance relationship to get the proper invalidation logic.

Fixes sbt/zinc#830.

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

@@ -306,6 +306,13 @@ private class ExtractDependenciesCollector extends tpd.TreeTraverser { thisTreeT
       }
     }
 
+  private def addInheritanceDependencies(tree: Closure)(using Context): Unit =
+    // If the tpt is empty, this is a non-SAM lambda, so no need to register
+    // an inheritance relationship.
+    if !tree.tpt.isEmpty then
+      val from = resolveDependencySource
+      _dependencies += ClassDependency(from, tree.tpt.tpe.classSymbol, LocalDependencyByInheritance)
+
   private def addInheritanceDependencies(tree: Template)(using Context): Unit =
     if (tree.parents.nonEmpty) {
       val depContext = depContextOf(tree.symbol.owner)
@@ -369,6 +376,8 @@ private class ExtractDependenciesCollector extends tpd.TreeTraverser { thisTreeT
       case ref: RefTree =>
         addMemberRefDependency(ref.symbol)
         addTypeDependency(ref.tpe)
+      case t: Closure =>
+        addInheritanceDependencies(t)
       case t: Template =>
         addInheritanceDependencies(t)
       case _ =>

sbt-test/source-dependencies/sam/A.scala

@@ -0,0 +1,3 @@
+trait A {
+  def foo(): Int
+}

sbt-test/source-dependencies/sam/B.scala

@@ -0,0 +1,2 @@
+class B:
+  val f: A = () => 1

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

@@ -0,0 +1 @@
+scalaVersion := sys.props("plugin.scalaVersion")

sbt-test/source-dependencies/sam/changes/A.scala

@@ -0,0 +1,3 @@
+trait A {
+  def foo(): String
+}

sbt-test/source-dependencies/sam/test

@@ -0,0 +1,7 @@
+> compile
+
+# change the SAM type
+$ copy-file changes/A.scala A.scala
+
+# Both A.scala and B.scala should be recompiled, producing a compile error
+-> compile