Fix meaning of this in a polymorphic function type

Author: smarter

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

@@ -22,7 +22,11 @@ trait TypeAssigner {
    */
   def qualifyingClass(tree: untpd.Tree, qual: Name, packageOK: Boolean)(using Context): Symbol = {
     def qualifies(sym: Symbol) =
-      sym.isClass && (
+      sym.isClass &&
+      // `this` in a polymorphic function type never refers to the desugared refinement.
+      // In other refinements, `this` does refer to the refinement but is deprecated
+      // (see `Checking#checkRefinementNonCyclic`).
+      !(sym.isRefinementClass && sym.derivesFrom(defn.PolyFunctionClass)) && (
           qual.isEmpty ||
           sym.name == qual ||
           sym.is(Module) && sym.name.stripModuleClassSuffix == qual)

tests/pos/polymorphic-functions-this.scala

@@ -0,0 +1,6 @@
+trait Foo:
+  type X
+  def x: X
+  val f: [T <: this.X] => (T, this.X) => (T, this.X) =
+    [T <: this.X] => (x: T, y: this.X) => (x, y)
+  f(x, x)