Document why patternMatcher needs to be after tailrec.

Author: DarkDimius

src/dotty/tools/dotc/Compiler.scala

@@ -57,7 +57,7 @@ class Compiler {
            new TailRec),
       List(new PatternMatcher,
            new ExplicitOuter,
-           new LazyValTranformContext().transformer, // disabled, awaiting fixes
+           new LazyValTranformContext().transformer,
            new Splitter),
       List(new ElimByName,
            new InterceptedMethods,

src/dotty/tools/dotc/transform/PatternMatcher.scala

@@ -12,6 +12,7 @@ import core.StdNames._
 import dotty.tools.dotc.ast.{TreeTypeMap, tpd}
 import dotty.tools.dotc.core
 import dotty.tools.dotc.core.DenotTransformers.DenotTransformer
+import dotty.tools.dotc.core.Phases.Phase
 import dotty.tools.dotc.core.{TypeApplications, Flags}
 import dotty.tools.dotc.typer.Applications
 import dotty.tools.dotc.util.Positions
@@ -41,6 +42,8 @@ class PatternMatcher extends MiniPhaseTransform with DenotTransformer {thisTrans
 
   override def runsAfter = Set(classOf[ElimRepeated])
 
+  override def runsAfterGroupsOf = Set(classOf[TailRec]) // tailrec is not capable of reversing the patmat tranformation made for tree
+
   override def phaseName = "patternMatcher"
 
   var _id = 0