From 532c2871f9c71eb2a39c431ec3616c7df889e92a Mon Sep 17 00:00:00 2001
From: Kacper Korban <kacper.f.korban@gmail.com>
Date: Tue, 16 Jul 2024 23:38:45 +0200
Subject: [PATCH] fix: Only implement a deferred given in a class if its parent
 won't implement it

---
 compiler/src/dotty/tools/dotc/typer/Typer.scala |  7 ++++++-
 tests/pos/i21189-alt.scala                      | 12 ++++++++++++
 tests/pos/i21189.scala                          | 10 ++++++++++
 3 files changed, 28 insertions(+), 1 deletion(-)
 create mode 100644 tests/pos/i21189-alt.scala
 create mode 100644 tests/pos/i21189.scala

diff --git a/compiler/src/dotty/tools/dotc/typer/Typer.scala b/compiler/src/dotty/tools/dotc/typer/Typer.scala
index c518de7dbbfe..821fd2e36b79 100644
--- a/compiler/src/dotty/tools/dotc/typer/Typer.scala
+++ b/compiler/src/dotty/tools/dotc/typer/Typer.scala
@@ -3038,7 +3038,7 @@ 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.
      */
@@ -3055,6 +3055,10 @@ 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)
@@ -3084,6 +3088,7 @@ 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)
diff --git a/tests/pos/i21189-alt.scala b/tests/pos/i21189-alt.scala
new file mode 100644
index 000000000000..08213cd627d4
--- /dev/null
+++ b/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]
diff --git a/tests/pos/i21189.scala b/tests/pos/i21189.scala
new file mode 100644
index 000000000000..88a0bf601476
--- /dev/null
+++ b/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]