Merge pull request #2617 from biboudis/fix-2468

Fix #2468: Replace wildcards in a mini-phase

Author: nicolasstucki

compiler/src/dotty/tools/dotc/Compiler.scala

@@ -99,7 +99,8 @@ class Compiler {
            new ElimStaticThis,      // Replace `this` references to static objects by global identifiers
            new Flatten,             // Lift all inner classes to package scope
            new RestoreScopes),      // Repair scopes rendered invalid by moving definitions in prior phases of the group
-      List(new MoveStatics,         // Move static methods to companion classes
+      List(new TransformWildcards,  // Replace wildcards with default values
+           new MoveStatics,         // Move static methods to companion classes
            new ExpandPrivate,       // Widen private definitions accessed from nested classes
            new SelectStatic,        // get rid of selects that would be compiled into GetStatic
            new CollectEntryPoints,  // Find classes with main methods

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

@@ -0,0 +1,30 @@
+package dotty.tools.dotc
+package transform
+
+import TreeTransforms._
+import core.DenotTransformers._
+import core.Contexts._
+import ast.tpd
+
+/** This phase transforms wildcards in valdefs with their default value.
+  *  In particular for every valdef that is declared:
+  *    `val x : T = _` to `val x : T = <zero of T>`
+  *
+  */
+class TransformWildcards extends MiniPhaseTransform with IdentityDenotTransformer { thisTransform =>
+  import tpd._
+
+  override def phaseName = "transformWildcards"
+
+  override def checkPostCondition(tree: Tree)(implicit ctx: Context): Unit = {
+    tree match {
+      case vDef: ValDef => assert(!tpd.isWildcardArg(vDef.rhs))
+      case _ =>
+    }
+  }
+
+  override def transformValDef(tree: ValDef)(implicit ctx: Context, info: TransformerInfo): Tree = {
+    if (ctx.owner.isClass) tree
+    else cpy.ValDef(tree)(rhs = tree.rhs.wildcardToDefault)
+  }
+}

tests/pos/i2468.scala

@@ -0,0 +1,27 @@
+
+object Test {
+
+  class A {
+    private[this] var x: String = _
+  }
+
+  class B {
+    private[this] var x: String = _
+    x = "foo"
+  }
+
+  class C {
+    private[this] var x1: Int = _
+    private[this] var x2: Unit = _
+    private[this] var x3: Char = _
+    private[this] var x4: Boolean = _
+    private[this] var x5: Float = _
+    private[this] var x6: Double = _
+    private[this] var x7: Char = _
+    private[this] var x8: Byte = _
+    private[this] var x9: AnyVal = _
+    private[this] var x10: D = _
+  }
+
+  class D(x: Int) extends AnyVal
+}