Don't null out private field in trait

Author: allanrenucci

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

@@ -33,12 +33,12 @@ object CollectNullableFields {
  *  initialised.
  *
  *  A field is nullable if all the conditions below hold:
+ *    - belongs to a non trait-class
  *    - is private
  *    - is not lazy
  *    - its type is nullable, or is an expression type (e.g. => Int)
  *    - is on used in a lazy val initializer
  *    - defined in the same class as the lazy val
- *    - TODO from Scalac? from a non-trait class
  */
 class CollectNullableFields extends MiniPhase {
   import tpd._
@@ -63,7 +63,11 @@ class CollectNullableFields extends MiniPhase {
     def isNullableType(tpe: Type) =
       tpe.isInstanceOf[ExprType] ||
       tpe.widenDealias.typeSymbol.isNullableClass
-    val isNullablePrivateField = sym.isField && sym.is(Private, butNot = Lazy) && isNullableType(sym.info)
+    val isNullablePrivateField =
+      sym.isField &&
+      sym.is(Private, butNot = Lazy) &&
+      !sym.owner.is(Trait) &&
+      isNullableType(sym.info)
 
     if (isNullablePrivateField)
       nullability.get(sym) match {

tests/run/i1692.scala

@@ -49,6 +49,11 @@ class LazyNotNullable {
   }
 }
 
+trait LazyTrait {
+  private val a = "A"
+  lazy val l0 = a
+}
+
 object Test {
   def main(args: Array[String]): Unit = {
     nullableTests()
@@ -111,6 +116,10 @@ object Test {
     val inner = new lz.Inner
     assert(inner.l8 == "H")
     assertNotNull("h")
+
+    val fromTrait = new LazyTrait {}
+    assert(fromTrait.l0 == "A")
+    assert(readField("LazyTrait$$a", fromTrait) != null) // fragile: test will break if compiler generated name change
   }
 
   def readField(fieldName: String, target: Any): Any = {