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Fix how trailing list subpatterns are cached #3278

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Aug 17, 2023
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Fix how trailing list subpatterns are cached #3278

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26 changes: 23 additions & 3 deletions accepted/3.0/patterns/feature-specification.md
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Expand Up @@ -4,7 +4,7 @@ Author: Bob Nystrom

Status: Accepted

Version 2.32 (see [CHANGELOG](#CHANGELOG) at end)
Version 2.33 (see [CHANGELOG](#CHANGELOG) at end)

Note: This proposal is broken into a couple of separate documents. See also
[records][] and [exhaustiveness][].
Expand Down Expand Up @@ -3389,8 +3389,9 @@ To bind invocation keys in a pattern `p` using parent invocation `i`:

3. Else `s` is a non-rest element after the rest element:

1. Let `e` be `i : ("tail[]", [index])` where `index` is the
zero-based index of this element subpattern.
1. Let `e` be `i : ("tail[]", [tailIndex])` where `tailIndex` is
the number of element subpatterns following this element
subpattern.

*Note the "tail" in the invocation key name. This is to
distinguish elements after a rest element at some position from
Expand All @@ -3409,6 +3410,20 @@ To bind invocation keys in a pattern `p` using parent invocation `i`:
the third element of the same list. So we use an invocation key
of "tail[]" for `c` and "[]" for `d`.*

*We use `tailIndex` and count backwards from the end so that
trailing elements can be cached across patterns, as in:*

```dart
switch (list) {
case [..., var a, var b]: ...
case [..., var c]: ...
}
```

*Here, `var b` and `var c` have the same `tailIndex` (0), so
the second case will use the previously cached list element
value for `var c`.(

2. Bind `e` to the `[]` invocation for `s`.

3. Bind invocations in the element subpattern using parent `e`.
Expand Down Expand Up @@ -3535,6 +3550,11 @@ Here is one way it could be broken down into separate pieces:

## Changelog

### 2.33 (after shipping)

- Tweak caching of trailing list elements after a rest element. The specified
behavior now follows the implementations (#2922).

### 2.32

- Prohibit variable and identifier patterns from being named `when` or `as`,
Expand Down