From ccde55e6271ad40715a46a42d228b669ea8e24d8 Mon Sep 17 00:00:00 2001
From: Hamza Remmal <hamza@remmal.net>
Date: Wed, 24 Sep 2025 17:32:32 +0200
Subject: [PATCH 1/2] Revert "fix: do not transform `Ident` to `This` in
 PostTyper anymore"

---
 .../src/dotty/tools/dotc/transform/PostTyper.scala    |  6 +++++-
 tests/run/i23875.check                                |  3 ---
 tests/run/i23875.scala                                | 11 -----------
 3 files changed, 5 insertions(+), 15 deletions(-)
 delete mode 100644 tests/run/i23875.check
 delete mode 100644 tests/run/i23875.scala

diff --git a/compiler/src/dotty/tools/dotc/transform/PostTyper.scala b/compiler/src/dotty/tools/dotc/transform/PostTyper.scala
index d1246aacf7d5..9f79c063dc03 100644
--- a/compiler/src/dotty/tools/dotc/transform/PostTyper.scala
+++ b/compiler/src/dotty/tools/dotc/transform/PostTyper.scala
@@ -391,7 +391,11 @@ class PostTyper extends MacroTransform with InfoTransformer { thisPhase =>
             checkNotPackage(tree)
           else
             registerNeedsInlining(tree)
-            checkUsableAsValue(tree)
+            val tree1 = checkUsableAsValue(tree)
+            tree1.tpe match {
+              case tpe: ThisType => This(tpe.cls).withSpan(tree.span)
+              case _ => tree1
+            }
         case tree @ Select(qual, name) =>
           registerNeedsInlining(tree)
           if name.isTypeName then
diff --git a/tests/run/i23875.check b/tests/run/i23875.check
deleted file mode 100644
index 36bc6136b8bf..000000000000
--- a/tests/run/i23875.check
+++ /dev/null
@@ -1,3 +0,0 @@
-true
-false
-false
diff --git a/tests/run/i23875.scala b/tests/run/i23875.scala
deleted file mode 100644
index 6993fb39f50b..000000000000
--- a/tests/run/i23875.scala
+++ /dev/null
@@ -1,11 +0,0 @@
-object Foo {
-  override def equals(that : Any) = that match {
-    case _: this.type => true
-    case _            => false
-  }
-}
-
-@main def Test =
-  println(Foo.equals(Foo))
-  println(Foo.equals(new AnyRef {}))
-  println(Foo.equals(0))

From 01eab3b9be7065cfc1fc241223b9b6ceea8dc9e6 Mon Sep 17 00:00:00 2001
From: Hamza Remmal <hamza@remmal.net>
Date: Wed, 24 Sep 2025 18:00:37 +0200
Subject: [PATCH 2/2] fix: make the pattern matcher understand this types

---
 .../dotty/tools/dotc/transform/PatternMatcher.scala   |  8 +++++++-
 tests/run/i23875.check                                |  3 +++
 tests/run/i23875.scala                                | 11 +++++++++++
 3 files changed, 21 insertions(+), 1 deletion(-)
 create mode 100644 tests/run/i23875.check
 create mode 100644 tests/run/i23875.scala

diff --git a/compiler/src/dotty/tools/dotc/transform/PatternMatcher.scala b/compiler/src/dotty/tools/dotc/transform/PatternMatcher.scala
index 825b7dc337b4..558cbd72dd43 100644
--- a/compiler/src/dotty/tools/dotc/transform/PatternMatcher.scala
+++ b/compiler/src/dotty/tools/dotc/transform/PatternMatcher.scala
@@ -482,7 +482,13 @@ object PatternMatcher {
               onSuccess
             )
           }
-        case WildcardPattern() =>
+        // When match against a `this.type` (say case a: this.type => ???),
+        // the typer will transform the pattern to a `Bind(..., Typed(Ident(a), ThisType(...)))`, 
+        // then post typer will change all the `Ident` with a `ThisType` to a `This`.
+        // Therefore, after pattern matching, we will have the following tree `Bind(..., Typed(This(...), ThisType(...)))`.
+        // We handle now here the case were the pattern was transformed to a `This`, relying on the fact that the logic for
+        // `Typed` above will create the correct type test.
+        case WildcardPattern() | This(_) =>
           onSuccess
         case SeqLiteral(pats, _) =>
           matchElemsPlan(scrutinee, pats, exact = true, onSuccess)
diff --git a/tests/run/i23875.check b/tests/run/i23875.check
new file mode 100644
index 000000000000..36bc6136b8bf
--- /dev/null
+++ b/tests/run/i23875.check
@@ -0,0 +1,3 @@
+true
+false
+false
diff --git a/tests/run/i23875.scala b/tests/run/i23875.scala
new file mode 100644
index 000000000000..6993fb39f50b
--- /dev/null
+++ b/tests/run/i23875.scala
@@ -0,0 +1,11 @@
+object Foo {
+  override def equals(that : Any) = that match {
+    case _: this.type => true
+    case _            => false
+  }
+}
+
+@main def Test =
+  println(Foo.equals(Foo))
+  println(Foo.equals(new AnyRef {}))
+  println(Foo.equals(0))