fix: make the pattern matcher understand this types

[Cherry-picked 01eab3b]

Author: hamzaremmal

compiler/src/dotty/tools/dotc/transform/PatternMatcher.scala

@@ -478,7 +478,13 @@ 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)

tests/run/i23875.check

@@ -0,0 +1,3 @@
+true
+false
+false

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))