A fix and a partially failing test

Author: odersky

compiler/src/dotty/tools/dotc/core/NamerOps.scala

@@ -14,14 +14,15 @@ object NamerOps:
    *  @param ctor the constructor
    */
   def effectiveResultType(ctor: Symbol, paramss: List[List[Symbol]])(using Context): Type =
-    paramss match
+    var resType = paramss match
       case TypeSymbols(tparams) :: _ =>
-        var resType = ctor.owner.typeRef.appliedTo(tparams.map(_.typeRef))
-        for params <- paramss; param <- params do
-          if param.is(Tracked) then
-            resType = RefinedType(resType, param.name, param.termRef)
-        resType
-      case _ => ctor.owner.typeRef
+        ctor.owner.typeRef.appliedTo(tparams.map(_.typeRef))
+      case _ =>
+        ctor.owner.typeRef
+    for params <- paramss; param <- params do
+      if param.is(Tracked) then
+        resType = RefinedType(resType, param.name, param.termRef)
+    resType
 
   /** If isConstructor, make sure it has at least one non-implicit parameter list
    *  This is done by adding a () in front of a leading old style implicit parameter,

tests/neg/i3964.scala

@@ -0,0 +1,32 @@
+trait Animal
+class Dog extends Animal
+class Cat extends Animal
+
+object Test1:
+
+  abstract class Bar { val x: Animal }
+  val bar: Bar { val x: Cat } = new Bar { val x = new Cat }  // error, but should work
+
+  trait Foo { val x: Animal }
+  val foo: Foo { val x: Cat } = new Foo { val x = new Cat }  // error, but should work
+
+object Test2:
+  abstract class Bar(tracked val x: Animal)
+  val b = new Bar(new Cat)
+  val bar: Bar { val x: Cat } = new Bar(new Cat)  // ok
+
+  trait Foo(tracked val x: Animal)
+  val foo: Foo { val x: Cat } = new Foo(new Cat)  // ok
+
+object Test3:
+  trait Vec(tracked val size: Int)
+  class Vec8 extends Vec(8)
+
+  abstract class Lst(tracked val size: Int)
+  class Lst8 extends Lst(8)
+
+  val v8a: Vec { val size: 8 } = new Vec8        // error, but should work
+  val v8b: Vec { val size: 8 } = new Vec(8)      // ok
+
+  val l8a: Lst { val size: 8 } = new Lst8        // error, but should work
+  val l8b: Lst { val size: 8 } = new Lst(8)      // ok