Type Constructor Inference fails to consider implemented traits

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trait T[A]
def usetHk[T[_], A](ta: T[A]) = 0
usetHk(new T[Int]{}: T[Int])
usetHk(new T[Int]{}) // fails with: found: java.lang.Object with T[Int], required: ?T[ ?A ]

Scala Version: 2.8.0.r19664-b20091116020221

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Imported From: https://issues.scala-lang.org/browse/SI-2693?orig=1
Reporter: @retronym

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@paulp said:
(In r23383) A double goodness whammy involving type inference at the borders.

1. Implicit search preserves singleton type fidelity.
2. Unification of parent bounds is (closer to) correct.

Result of 1: "implicit def f(x: Foo.type)" will convert object Foo.
Result of 2: "new Trait[Int] { }" may enjoy its type constructor being
inferred, no longer foiled by the anonymous class.

Also included are some clarity-enhnancing renamings and refactorings.
Performance note: I heavily benchmarked the change to isSubArgs and it
is reproducibly faster than the previous implementation. Numbers
and methodology available upon request.

Closes #2693, #3964. Review by moors, who wrote most of this patch
but might like to review the comments.

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@retronym said:
Thanks guys, this should reduce the boilerplate needed to declare type class instances.

While you're in a type unifying frenzy, I'll submit my Christmas wishlist: http://gist.github.com/650709 (Basically #2712, with the concession that the inference could be given a helping hand by the type of the implicit args)