On Wednesday, 9 August 2017 at 21:54:46 UTC, Q. Schroll wrote:
For a class/interface type `A` and a class `C` inheriting from `A` one can do

  A a = getA();
  if (auto c = cast(C) a)
  { .. use c .. }

to get a `C` view on `a` if it happens to be a `C`-instance.

Sometimes one cannot find a good new name for `c` while there is no advantage of accessing `a` when `c` is available. D does not allow to shadow `a` in the if-auto declaration for good reasons. How about relaxing the rule for cases like these, where the rhs is the lhs with a cast to derived?

  if (auto a = cast(C) a)
  { .. use a typed as C .. }

One can think of `a` being *statically* retyped to `C` as this is a (strictly) better type information. Internally, it would be a shadowing, but it does not matter as the disadvantages don't apply (if I didn't miss something).

One option is to use https://dlang.org/library/std/algorithm/comparison/cast_switch.html

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