Use EmptyTree for the cond of infinite WhileDo loops.

Such `WhileDo` node have type `Nothing` instead of `Unit`. This is
used in the loops generated by `TailRec` in order not to need some
spurious dead code.

Author: sjrd

compiler/src/dotty/tools/dotc/transform/TailRec.scala

@@ -141,14 +141,12 @@ class TailRec extends MiniPhase {
             }
 
             Block(
-              initialValDefs :::
-              WhileDo(Literal(Constant(true)), {
+              initialValDefs,
+              WhileDo(EmptyTree, {
                 Labeled(transformer.continueLabel.get.asTerm, {
                   Return(rhsFullyTransformed, ref(origMeth))
                 })
-              }) ::
-              Nil,
-              Throw(Literal(Constant(null))) // unreachable code
+              })
             )
           } else {
             if (mandatory) ctx.error(

compiler/src/dotty/tools/dotc/typer/TypeAssigner.scala

@@ -487,7 +487,7 @@ trait TypeAssigner {
     tree.withType(defn.NothingType)
 
   def assignType(tree: untpd.WhileDo)(implicit ctx: Context) =
-    tree.withType(defn.UnitType)
+    tree.withType(if (tree.cond eq EmptyTree) defn.NothingType else defn.UnitType)
 
   def assignType(tree: untpd.Try, expr: Tree, cases: List[CaseDef])(implicit ctx: Context) =
     if (cases.isEmpty) tree.withType(expr.tpe)

compiler/src/dotty/tools/dotc/typer/Typer.scala

@@ -1138,7 +1138,9 @@ class Typer extends Namer
   }
 
   def typedWhileDo(tree: untpd.WhileDo)(implicit ctx: Context): Tree = track("typedWhileDo") {
-    val cond1 = typed(tree.cond, defn.BooleanType)
+    val cond1 =
+      if (tree.cond eq EmptyTree) EmptyTree
+      else typed(tree.cond, defn.BooleanType)
     val body1 = typed(tree.body, defn.UnitType)
     assignType(cpy.WhileDo(tree)(cond1, body1))
   }