Fix trait parameter passing algorithm

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

@@ -258,7 +258,7 @@ class Mixin extends MiniPhase with SymTransformer { thisPhase =>
            && !wasOneOf(getter, Deferred)
            && !getter.isConstExprFinalVal
       yield
-        if (isCurrent(getter) || getter.name.is(ExpandedName)) {
+        if (isInImplementingClass(getter) || getter.name.is(ExpandedName)) {
           val rhs =
             if (wasOneOf(getter, ParamAccessor))
               nextArgument()
@@ -271,6 +271,9 @@ class Mixin extends MiniPhase with SymTransformer { thisPhase =>
           // transformFollowing call is needed to make memoize & lazy vals run
           transformFollowing(DefDef(mkForwarderSym(getter.asTerm), rhs))
         }
+        else if wasOneOf(getter, ParamAccessor) then
+          // mixin parameter field is defined by an override; evaluate the argument and throw it away
+          nextArgument()
         else EmptyTree
     }
 

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

@@ -42,7 +42,7 @@ class MixinOps(cls: ClassSymbol, thisPhase: DenotTransformer)(using Context) {
   /** Is `sym` a member of implementing class `cls`?
    *  The test is performed at phase `thisPhase`.
    */
-  def isCurrent(sym: Symbol): Boolean =
+  def isInImplementingClass(sym: Symbol): Boolean =
     atPhase(thisPhase) {
       cls.info.nonPrivateMember(sym.name).hasAltWith(_.symbol == sym)
     }
@@ -71,7 +71,7 @@ class MixinOps(cls: ClassSymbol, thisPhase: DenotTransformer)(using Context) {
     meth.is(Method, butNot = PrivateOrAccessorOrDeferred) &&
     (ctx.settings.mixinForwarderChoices.isTruthy || meth.owner.is(Scala2x) || needsDisambiguation || hasNonInterfaceDefinition ||
      generateJUnitForwarder || generateSerializationForwarder) &&
-    isCurrent(meth)
+    isInImplementingClass(meth)
   }
 
   final val PrivateOrAccessor: FlagSet = Private | Accessor

docs/docs/reference/other-new-features/trait-parameters.md

@@ -26,14 +26,12 @@ class D extends C, Greeting("Bill") // error: parameter passed twice
 Should this print "Bob" or "Bill"? In fact this program is illegal,
 because it violates the second rule of the following for trait parameters:
 
- 1. If a class `C` directly extends a parameterized trait `T`, and its superclass does not, `C` _must_ pass arguments to `T`.
+ 1. If a class `C` extends a parameterized trait `T`, and its superclass does not, `C` _must_ pass arguments to `T`.
 
- 2. If a class `C` directly or indirectly extends a parameterized trait `T`, and its superclass does as well, `C` _must not_  pass arguments to `T`.
+ 2. If a class `C` extends a parameterized trait `T`, and its superclass does as well, `C` _must not_  pass arguments to `T`.
 
  3. Traits must never pass arguments to parent traits.
 
- 4. If a class `C` extends a parameterized trait `T` only indirectly, and its superclass does not extend `T`, then all parameters of `T` must be defined via overrides.
-
 Here's a trait extending the parameterized trait `Greeting`.
 
 ```scala
@@ -53,13 +51,6 @@ The correct way to write `E` is to extend both `Greeting` and
 ```scala
 class E extends Greeting("Bob"), FormalGreeting
 ```
-Alternatively, a class could also define the `name` parameter of `Greeting` using
-an override, using rule (4) above:
-
-```scala
-class E2 extends FormalGreeting:
-   override val name: String = "Bob"
-```
 
 ## Reference
 

tests/pos/i11214.scala → tests/neg/i11214.scala

@@ -1,5 +1,5 @@
 trait Pet(val name: String)
 trait FeatheredPet extends Pet
 
-class Bird(override val name: String) extends FeatheredPet:
+class Bird(override val name: String) extends FeatheredPet: // error
    override def toString = s"bird name: $name"

tests/pos/i11344.scala

@@ -0,0 +1,8 @@
+trait Pet(val name: String, rest: Int):
+   def f(suffix: String) = s"$name$suffix$rest"
+
+class Birdie(override val name: String) extends Pet("huh", 1)
+
+@main def Test =
+  assert(Birdie("Polly").f("more") == "Pollymore1")
+