From faf8716f3d392d3597fa0c3ec359223a255d4b4b Mon Sep 17 00:00:00 2001
From: Martin Odersky <odersky@gmail.com>
Date: Sun, 7 Feb 2021 11:42:38 +0100
Subject: [PATCH 1/2] Allow to define trait parameters via overrides

This legalizes the behavior exhibited in #11214.

Fixes #11214
---
 .../other-new-features/trait-parameters.md          | 13 +++++++++++--
 tests/pos/i11214.scala                              |  5 +++++
 2 files changed, 16 insertions(+), 2 deletions(-)
 create mode 100644 tests/pos/i11214.scala

diff --git a/docs/docs/reference/other-new-features/trait-parameters.md b/docs/docs/reference/other-new-features/trait-parameters.md
index 1655e338b32c..f930c10e2c5d 100644
--- a/docs/docs/reference/other-new-features/trait-parameters.md
+++ b/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
 
diff --git a/tests/pos/i11214.scala b/tests/pos/i11214.scala
new file mode 100644
index 000000000000..0f34dbce4bbe
--- /dev/null
+++ b/tests/pos/i11214.scala
@@ -0,0 +1,5 @@
+trait Pet(val name: String)
+trait FeatheredPet extends Pet
+
+class Bird(name: String) extends FeatheredPet:
+   override def toString = s"bird name: $name"
\ No newline at end of file

From 0823a8f0a36c8adcb760bf3e293cb41ae14b36eb Mon Sep 17 00:00:00 2001
From: Martin Odersky <odersky@gmail.com>
Date: Sun, 7 Feb 2021 18:46:33 +0100
Subject: [PATCH 2/2] Fix test

---
 tests/pos/i11214.scala | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/tests/pos/i11214.scala b/tests/pos/i11214.scala
index 0f34dbce4bbe..154f203b7e20 100644
--- a/tests/pos/i11214.scala
+++ b/tests/pos/i11214.scala
@@ -1,5 +1,5 @@
 trait Pet(val name: String)
 trait FeatheredPet extends Pet
 
-class Bird(name: String) extends FeatheredPet:
+class Bird(override val name: String) extends FeatheredPet:
    override def toString = s"bird name: $name"
\ No newline at end of file