Extract variable refactor should create variables inside of switch statement

[stereotype441]

Given the following Dart 3.0 code:

test(Object o) {
  switch (o) {
    case int():
      print(o + 1);
  }
}

If I select the text o + 1 and perform an "extract variable" refactor, I get:

test(Object o) {
  var object = o + 1;
  switch (o) {
    case int():
      print(object);
  }
}

Which has a compile time error, because at the site of var object = o + 1; the variable o is not known to be an int.

I would have expected this instead:

test(Object o) {
  switch (o) {
    case int():
      var object = o + 1;
      print(object);
  }
}

[bwilkerson]

I agree. I would have expected the same behavior in pre-3.0 code. The variable should be declared as close as possible to the use site, typically on the preceding line.

But the example did make me curious about how we handle switch expressions, so I tried

int f(Object o) {
  return switch (o) {
    int() => o + 1,
    _ => 0,
  };
}

and found that it (not surprisingly) defines the variable before the return. It isn't valid to declare the variable in the expression, so I'm wondering whether we ought to just disable the assist in switch expressions (unless we can prove that the code to be generated would be error-free).

[scheglov]

https://dart-review.googlesource.com/c/sdk/+/295100