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

[swift-ci]

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 Missing @warn_unused_result attributes in Standard Library

Issue Description:

Try the following code:

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.