Open Radar

 
Summary:
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
  
  print(error)
}

print(error)
```

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.

Regression:
None.

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