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
    }

2 files changed, 28 insertions, 0 deletions

diff --git a/test/files/neg/t6443c.check b/test/files/neg/t6443c.check
new file mode 100644
index 0000000000..7cf8d23f4b
--- /dev/null
+++ b/test/files/neg/t6443c.check
@@ -0,0 +1,7 @@
+t6443c.scala:16: error: double definition:
+method foo:(d: B.D)(a: Any)(d2: d.type)Unit and
+method foo:(d: B.D)(a: Any, d2: d.type)Unit at line 11
+have same type after erasure: (d: B.D, a: Object, d2: B.D)Unit
+  def foo(d: D)(a: Any)(d2: d.type): Unit = ()
+      ^
+one error found
diff --git a/test/files/neg/t6443c.scala b/test/files/neg/t6443c.scala
new file mode 100644
index 0000000000..817224e043
--- /dev/null
+++ b/test/files/neg/t6443c.scala
@@ -0,0 +1,21 @@
+trait A {
+  type D >: Null <: C
+  def foo(d: D)(a: Any, d2: d.type): Unit
+  trait C {
+    def bar: Unit = foo(null)(null, null)
+  }
+}
+object B extends A {
+  class D extends C
+
+  def foo(d: D)(a: Any, d2: d.type): Unit = () // Bridge method required here!
+
+  // No bridge method should be added, but we'll be happy enough if
+  // the "same type after erasure" error kicks in before the duplicated
+  // bridge causes a problem.
+  def foo(d: D)(a: Any)(d2: d.type): Unit = ()
+}
+
+object Test extends App {
+  new B.D().bar
+}