Xcode-7 (7A120f): «Hidden» variable name on exhaustive `catch` block

Number:rdar://21396321 Date Originated:15-Jun-2015 09:14 PM
Status:Closed Resolved:
Product:Developer Tools Product Version:Xcode-7 (7A120f)
Classification:Serious Bug Reproducible:Always
When writing a `do`/`catch` block with an exhaustive `catch` `Swift` is adding a hidden `let` called `error` that limits the definition of a variable with that name outside of the calling block.

Steps to Reproduce:
Copy the following code to a `Playground`:

enum TestError: ErrorType {
  case HiddenErrorVariable

func throwingFunction() throws -> () {
  throw TestError.HiddenErrorVariable

let error: TestError
do {
  try throwingFunction()
} catch {
  error = TestError.HiddenErrorVariable


Expected Results:
Inside the `catch` block I should be able to assign to my own `error` variable because is an exhaustive `catch`.

Actual Results:
`Swift` «hijacks» the `error` variable inside the exhaustive `catch` instead of using a `_` as a `hidden` variable name.


`Swift` shouldn’t be naming `hidden` variables and instead should use of the `_` character when «needing» to have some «unused» parameter.


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