[SR-778] Forgetting the in keyword can sometimes lead to compiling code

| | |
|------------------|-----------------|
|Previous ID | SR-778 |
|Radar | None |
|Original Reporter | chriseidhof (JIRA User) |
|Type | Bug |
|Status | Reopened |
|Resolution | |


Additional Detail from JIRA

| | |
|------------------|-----------------|
|Votes | 0 |
|Component/s | Standard Library |
|Labels | Bug |
|Assignee | None |
|Priority | Medium |

md5: 0093a5f05b0f3ded251f13fdd054fe04

**relates to**:
* [SR-245](https://bugs.swift.org/browse/SR-245) Missing @warn_unused_result attributes in Standard Library

**Issue Description:**

Try the following code:

```swift
var x = 0
let capture: () -> Int = { [x]
return x
}
x++
capture()
```

Expected result: because there is a capture list `[x]` that should capture x by value, we expect the result of the final call to be 0. However, it is 1. This is because we forget the in keyword, and `[x]` gets parsed as an array literal.

A possible solution would be to warn about `[x]` being an unused expression.