From 2c2f41d8856eac89cc822944424aa8de0d7c15f0 Mon Sep 17 00:00:00 2001
From: "copilot-swe-agent[bot]" <198982749+Copilot@users.noreply.github.com>
Date: Fri, 29 Aug 2025 13:08:31 +0000
Subject: [PATCH 1/4] Initial plan


From bafdaf2b9c810d2e5465c3b524cf5b5bb7ba48dc Mon Sep 17 00:00:00 2001
From: "copilot-swe-agent[bot]" <198982749+Copilot@users.noreply.github.com>
Date: Fri, 29 Aug 2025 13:18:21 +0000
Subject: [PATCH 2/4] Initial analysis of issue #22310 - override checking bug

Co-authored-by: noti0na1 <8036790+noti0na1@users.noreply.github.com>
---
 sbt | 0
 1 file changed, 0 insertions(+), 0 deletions(-)
 create mode 100644 sbt

diff --git a/sbt b/sbt
new file mode 100644
index 000000000000..e69de29bb2d1

From 42468890bed6c1bd34d6d9531a4ec6aea91808fb Mon Sep 17 00:00:00 2001
From: "copilot-swe-agent[bot]" <198982749+Copilot@users.noreply.github.com>
Date: Fri, 29 Aug 2025 13:25:03 +0000
Subject: [PATCH 3/4] Identify root cause of issue #22310 in type signature
 matching

Co-authored-by: noti0na1 <8036790+noti0na1@users.noreply.github.com>
---
 tests/neg/i22310.scala | 28 ++++++++++++++++++++++++++++
 1 file changed, 28 insertions(+)
 create mode 100644 tests/neg/i22310.scala

diff --git a/tests/neg/i22310.scala b/tests/neg/i22310.scala
new file mode 100644
index 000000000000..f8854f0e6bc7
--- /dev/null
+++ b/tests/neg/i22310.scala
@@ -0,0 +1,28 @@
+//> using options -Xfatal-warnings
+
+// This should fail compilation but currently passes
+abstract class A0[T] {
+  def func(arg0: A0[String], arg1: T): Unit
+}
+
+abstract class A1 extends A0[String] {
+  override def func(arg0: A0[Object], arg1: String): Unit = {} // error: overrides nothing
+}
+
+// Similar issue with multiple type parameters
+abstract class B0[T, U] {
+  def method(x: B0[String, Int], y: T, z: U): Unit
+}
+
+abstract class B1 extends B0[String, Int] {
+  override def method(x: B0[Object, Int], y: String, z: Int): Unit = {} // error: overrides nothing
+}
+
+// This should work (correct override)
+abstract class C0[T] {
+  def goodFunc(arg0: C0[String], arg1: T): Unit
+}
+
+abstract class C1 extends C0[String] {
+  override def goodFunc(arg0: C0[String], arg1: String): Unit = {} // OK - this should work
+}
\ No newline at end of file

From 9b5a23b70b9a0a76523cc0d6cb08c5049dab670d Mon Sep 17 00:00:00 2001
From: "copilot-swe-agent[bot]" <198982749+Copilot@users.noreply.github.com>
Date: Fri, 29 Aug 2025 13:27:34 +0000
Subject: [PATCH 4/4] Fix issue #22310: Use strict type matching for override
 checks

Co-authored-by: noti0na1 <8036790+noti0na1@users.noreply.github.com>
---
 .../dotty/tools/dotc/typer/RefChecks.scala    |  6 ++++-
 tests/neg/i22310.check                        |  8 ++++++
 tests/pos/i22310-valid.scala                  | 27 +++++++++++++++++++
 3 files changed, 40 insertions(+), 1 deletion(-)
 create mode 100644 tests/neg/i22310.check
 create mode 100644 tests/pos/i22310-valid.scala

diff --git a/compiler/src/dotty/tools/dotc/typer/RefChecks.scala b/compiler/src/dotty/tools/dotc/typer/RefChecks.scala
index a79408b756ee..1043451e3080 100644
--- a/compiler/src/dotty/tools/dotc/typer/RefChecks.scala
+++ b/compiler/src/dotty/tools/dotc/typer/RefChecks.scala
@@ -974,7 +974,11 @@ object RefChecks {
 
       def isSignatureMatch(sym: Symbol) = sym.isType || {
         val self = clazz.thisType
-        sym.asSeenFrom(self).matches(member.asSeenFrom(self))
+        val symDenotation = sym.asSeenFrom(self)
+        val memberDenotation = member.asSeenFrom(self)
+        // Use matchesLoosely with alwaysCompareTypes = true to ensure strict type checking for overrides
+        // This fixes issue #22310 where A0[String] was incorrectly considered to match A0[Object]
+        symDenotation.matchesLoosely(memberDenotation, alwaysCompareTypes = true)
         && !incompatibleRepeatedParam(sym, member)
       }
 
diff --git a/tests/neg/i22310.check b/tests/neg/i22310.check
new file mode 100644
index 000000000000..f64557c5f7c7
--- /dev/null
+++ b/tests/neg/i22310.check
@@ -0,0 +1,8 @@
+-- [E177] Syntax Error: tests/neg/i22310.scala:9:2 --------------------------------
+9 |  override def func(arg0: A0[Object], arg1: String): Unit = {} // error: overrides nothing
+  |  ^
+  |  override def func overrides nothing.
+-- [E177] Syntax Error: tests/neg/i22310.scala:16:2 -------------------------------
+16|  override def method(x: B0[Object, Int], y: String, z: Int): Unit = {} // error: overrides nothing
+  |  ^
+  |  override def method overrides nothing.
\ No newline at end of file
diff --git a/tests/pos/i22310-valid.scala b/tests/pos/i22310-valid.scala
new file mode 100644
index 000000000000..c91fcb4afc46
--- /dev/null
+++ b/tests/pos/i22310-valid.scala
@@ -0,0 +1,27 @@
+// Test cases to ensure valid overrides still work after fixing #22310
+
+abstract class ValidCase1[T] {
+  def method(arg: ValidCase1[String], param: T): Unit
+}
+
+abstract class ValidCase1Sub extends ValidCase1[String] {
+  override def method(arg: ValidCase1[String], param: String): Unit = {} // OK - this should work
+}
+
+// Variance cases should still work correctly
+abstract class CovariantCase[+T] {
+  def method(param: CovariantCase[String]): T
+}
+
+abstract class CovariantCaseSub extends CovariantCase[Object] {
+  override def method(param: CovariantCase[String]): Object = "valid" // OK - covariance in return type
+}
+
+// Test with concrete classes too
+class ConcreteBase[T] {
+  def func(x: ConcreteBase[String], y: T): Unit = {}
+}
+
+class ConcreteDerived extends ConcreteBase[String] {
+  override def func(x: ConcreteBase[String], y: String): Unit = {} // OK - this should work
+}
\ No newline at end of file