Support param accessors that override concrete trait vals.

Author: sjrd

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

@@ -150,6 +150,12 @@ class Memoize extends MiniPhase with IdentityDenotTransformer { thisPhase =>
           // When transforming the getter, we determined that no field was needed.
           // In that case we can keep the setter as is, with a () rhs.
           tree
+        else if field.getter.is(ParamAccessor, butNot = Mutable) then
+          // This is a trait setter (because not Mutable) for a param accessor.
+          // We must keep the () rhs of the trait setter, otherwise the value
+          // inherited from the trait will overwrite the value of the parameter.
+          // See tests/run/traitValOverriddenByParamAccessor.scala
+          tree
         else
           field.setFlag(Mutable) // Necessary for vals mixed in from traits
           val initializer =

tests/run/traitValOverriddenByParamAccessor.scala

@@ -0,0 +1,17 @@
+trait Foo {
+  val someField: Int = 5
+}
+
+class SimpleFoo extends Foo
+
+class Bar(override val someField: Int) extends Foo
+
+object Test {
+  def main(args: Array[String]): Unit = {
+    val simpleFoo = new SimpleFoo
+    assert(simpleFoo.someField == 5)
+
+    val bar = new Bar(44)
+    assert(bar.someField == 44)
+  }
+}