Fix erasure of curried polymorphic function types

Fixes #6678
Fixes #8921
Fixes #13304
Fixes #13426

Author: odersky

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

@@ -700,7 +700,11 @@ object Erasure {
           // PolyFunction apply Selects will not have a symbol, so deduce the owner
           // from the typed qual.
           val owner = qual1.tpe.typeSymbol
-          if defn.isFunctionClass(owner) then owner else NoSymbol
+          if defn.isFunctionClass(owner) && false then owner
+          else
+            val alt = erasure(tree.qualifier.typeOpt).typeSymbol
+            if defn.isFunctionClass(alt) then alt
+            else NoSymbol
         else
           val owner = sym.maybeOwner
           if defn.specialErasure.contains(owner) then

tests/pos/i13426.scala

@@ -0,0 +1,2 @@
+val f = [R] => () => [A] => (a: A => R, b: A) => a(b)
+val x = f[Int]()(_, 3)

tests/pos/i6678.scala

@@ -0,0 +1,4 @@
+object O {
+  val f = (i:Int) => [T] => (s:String) => 2
+  def m = f(1)("a")
+}

tests/pos/i8921.scala

@@ -0,0 +1,7 @@
+type R[F[_], A] =[B] => (A => F[B]) => F[B]
+
+type M[F[_]] =[A, B] => (A => F[B]) => F[A] => F[B]
+
+def mr[F[_]]: M[[A] =>> R[F, A]] =
+   [A, B] => (f: A => R[F, B]) => (m: R[F, A]) =>
+      [C] => (k: B => F[C]) => m(a => f(a)(k))

tests/run/i13304.check

@@ -0,0 +1 @@
+foo

tests/run/i13304.scala

@@ -0,0 +1,13 @@
+trait Zero[F[_]]:
+  def zero[A]: F[A]
+
+given Zero[List] with
+  def zero[A] = List.empty[A]
+
+given Zero[Option] with
+  def zero[A] = Option.empty[A]
+
+
+@main def Test =
+  val test = [F[_]] => (f: Zero[F]) ?=> [G[_]] => (g: Zero[G]) ?=> println("foo")
+  test[List][Option]