fix: Only implement a deferred given in a class if its parent won't implement it (#21206)

It should be possible to extend a class that inherits a deferred given,
so a generated given implementation should not be final.

closes #21189

Author: odersky

compiler/src/dotty/tools/dotc/typer/Typer.scala

@@ -3044,7 +3044,7 @@ class Typer(@constructorOnly nestingLevel: Int = 0) extends Namer
           body
 
     /** Implement givens that were declared with a `deferred` rhs.
-     *  The a given value matching the declared type is searched in a
+     *  The given value matching the declared type is searched in a
      *  context directly enclosing the current class, in which all given
      *  parameters of the current class are also defined.
      */
@@ -3061,6 +3061,10 @@ class Typer(@constructorOnly nestingLevel: Int = 0) extends Namer
             false
           else true
 
+        def willBeimplementedInParentClass(m: TermRef) =
+          val superCls = cls.superClass
+          superCls.exists && superCls.asClass.baseClasses.contains(m.symbol.owner)
+
         def givenImpl(mbr: TermRef): ValDef =
           val dcl = mbr.symbol
           val target = dcl.info.asSeenFrom(cls.thisType, dcl.owner)
@@ -3090,6 +3094,7 @@ class Typer(@constructorOnly nestingLevel: Int = 0) extends Namer
           cls.thisType.implicitMembers
             //.showing(i"impl def givens for $cls/$result")
             .filter(_.symbol.isAllOf(DeferredGivenFlags, butNot = Param))
+            .filter(!willBeimplementedInParentClass(_)) // only implement the given in the topmost class
             //.showing(i"impl def filtered givens for $cls/$result")
             .filter(isGivenValue)
             .map(givenImpl)

tests/pos/i21189-alt.scala

@@ -0,0 +1,12 @@
+//> using options -source:future -language:experimental.modularity
+
+class MySortedSet[T : Ord] extends SortedSet[T]
+
+trait Ord[T]
+
+trait Sorted[T] extends ParentOfSorted[T]
+
+trait ParentOfSorted[T]:
+  given Ord[T] as ord = compiletime.deferred
+
+class SortedSet[T : Ord] extends Sorted[T]

tests/pos/i21189.scala

@@ -0,0 +1,10 @@
+//> using options -source:future -language:experimental.modularity
+
+class MySortedSet[T : Ord] extends SortedSet[T]
+
+trait Ord[T]
+
+trait Sorted[T]:
+  given Ord[T] as ord = compiletime.deferred
+
+class SortedSet[T : Ord] extends Sorted[T]