Loosing type info depending on iteration mode

[xwu (Xiaodi Wu)]

For-in vs forEach() when sequence element is Dictionary.Key

First off, you probably know this, but both f1 and f2 should really take arguments of type some Sequence<Key>, not any, which would make the error go away. As I understand it, this is a known limitation that could be lifted in the future. Let me try to break it down. Starting with f2: forEach can preserve the concrete Key type thanks to Covariant Erasure with Constrained Existentials in SE-0353: Constrained Existential Types. So this is relatively straightforward. In the case of f1, the com…