SI-6443 Widen dependent param types in uncurry

Bridge building operates on unusual method signatures:
after uncurry, so parameter lists are collapsed; but before
erasure, so dependently typed parameters are still around.

Original:

  def foo(a: T)(b: a.type, c: a.U): Unit

During computeBridges:

  (a: T, b: a.type, c: a.U)Unit

This signature no longer appears to override the corresponding
one in a superclass, because the types of `b` and `c` are dependent
on method parameters.

The root of the problem is uncurry, which leaves the trees in
a poor state. This commit changes uncurry to remedy this.

An example illustrates it best:

    // source
    def foo(a: A)(b: a.type): b.type = b

    // post uncurry before this patch.
    // not well typed code!
    def foo(a: A, b: a.type): a.type = {

    // post uncurry after this patch
    def foo(a: A, b: A): A = {
       val b$1 = b.asInstanceOf[a.type]
       b$1
    }

1 files changed, 15 insertions, 0 deletions

diff --git a/test/files/run/t6443.scala b/test/files/run/t6443.scala
new file mode 100644
index 0000000000..67fe2cab22
--- /dev/null
+++ b/test/files/run/t6443.scala
@@ -0,0 +1,15 @@
+class Base
+class Derived extends Base
+
+trait A {
+  def foo(d: String)(d2: d.type): Base
+  val s = ""
+  def bar: Unit = foo(s)(s)
+}
+object B extends A {
+  def foo(d: String)(d2: d.type): D forSome { type D <: S; type S <: Derived } = {d2.isEmpty; null} // Bridge method required here!
+}
+
+object Test extends App {
+  B.bar
+}