Merge pull request #11338 from dotty-staging/fix-11214

Allow to define trait parameters via overrides

Author: odersky

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

@@ -26,12 +26,14 @@ 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` extends a parameterized trait `T`, and its superclass does not, `C` _must_ pass arguments to `T`.
+ 1. If a class `C` directly extends a parameterized trait `T`, and its superclass does not, `C` _must_ 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`.
+ 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`.
 
  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
@@ -51,6 +53,13 @@ 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

@@ -0,0 +1,5 @@
+trait Pet(val name: String)
+trait FeatheredPet extends Pet
+
+class Bird(override val name: String) extends FeatheredPet:
+   override def toString = s"bird name: $name"