fix: synthesise value of this.type in ValueOf

[Cherry-picked dc19b2f]

Author: hamzaremmal

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

@@ -231,6 +231,8 @@ class Synthesizer(typer: Typer)(using @constructorOnly c: Context):
             withNoErrors(success(Literal(Constant(()))))
           case n: TermRef =>
             withNoErrors(success(ref(n)))
+          case ts: ThisType =>
+            withNoErrors(success(This(ts.cls)))
           case tp =>
             EmptyTreeNoError
       case _ =>

tests/pos/i23086.scala

@@ -0,0 +1,2 @@
+class Test:
+  val _: this.type = summon[ValueOf[this.type]].value