Fix regression in phase assembly with consecutive runsRightAfter

If a phase `runsRightAfter` a phase that itself has been merged
into `Node` of the phase that _it_ `runsRightAfter`, we need to
find that `Node` to merge into.

Author: retronym

src/compiler/scala/tools/nsc/PhaseAssembly.scala

@@ -111,8 +111,11 @@ trait PhaseAssembly {
       for (node <- nodes.valuesIterator.toList) {
         val hardBefores = node.before.iterator.filter(_.hard).toList
         for (hl <- hardBefores) {
-          node.phaseobj = Some(node.phaseobj.get ++ hl.frm.phaseobj.get)
-          node.before = hl.frm.before
+          val effectiveNode: Node = if (nodes.contains(node.name)) node else {
+            nodes.find(_._2.phaseobj.exists(_.exists(_.phaseName == node.name))).get._2
+          }
+          effectiveNode.phaseobj = Some(effectiveNode.phaseobj.get ++ hl.frm.phaseobj.get)
+          effectiveNode.before = hl.frm.before
           nodes -= hl.frm.phasename
           edges -= hl
         }

test/junit/scala/tools/nsc/PhaseAssemblyTest.scala

@@ -0,0 +1,42 @@
+/*
+ * Scala (https://www.scala-lang.org)
+ *
+ * Copyright EPFL and Lightbend, Inc.
+ *
+ * Licensed under Apache License 2.0
+ * (http://www.apache.org/licenses/LICENSE-2.0).
+ *
+ * See the NOTICE file distributed with this work for
+ * additional information regarding copyright ownership.
+ */
+
+package scala.tools.nsc
+
+import org.junit.Assert.assertEquals
+import org.junit.Test
+
+class PhaseAssemblyTest {
+  @Test
+  def multipleRunsRightAfter(): Unit = {
+    val global = new Global(new Settings)
+    case class component[G <: Global with Singleton](global: G, phaseName: String, override val runsRightAfter: Option[String], override val runsAfter: List[String], override val runsBefore: List[String]) extends SubComponent {
+      override def newPhase(prev: Phase): Phase = ???
+    }
+    val N = 16
+    val random = new scala.util.Random(123502L)
+    val names = Array.fill(N)("phase_" + random.nextInt(1024))
+    val parserAndTerminal = List(
+      component(global, "parser", None, Nil, Nil),
+      component(global,"terminal", None, Nil, List(N.toString))
+    )
+    val components = List.tabulate(N)(i => component(global, names(i), Some(if (i == 0) "parser" else names(i - 1)), Nil, List("terminal"))) ::: parserAndTerminal
+
+    val graph = global.phasesSetToDepGraph(components.reverse)
+    graph.removeDanglingNodes()
+    graph.collapseHardLinks()
+    graph.assignLevelsAndDetectCycles(graph.getNodeByPhase("parser"))
+    val result: List[String] =graph.compilerPhaseList().map(_.phaseName).filter(_.startsWith("phase_"))
+    assertEquals(names.toList, result)
+  }
+
+}