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[SR-2989] Multi-line function chains evaluated before breakpoint hits #4581

swift-ci opened this issue Oct 19, 2016 · 3 comments
bug Something isn't working LLDB for Swift


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Previous ID SR-2989
Radar rdar://28883410
Original Reporter rosslebeau (JIRA User)
Type Bug
Status Resolved
Resolution Won't Do

Swift 3.0
macOS 10.12 16A323

Additional Detail from JIRA
Votes 1
Component/s LLDB for Swift
Labels Bug
Assignee @vedantk
Priority Medium

md5: b5bc35bd9978fec0d64c651b37689c3f

Issue Description:

In certain cases, if you have a multi-line function chain, placing a breakpoint on the first line will actually result in the function chain being evaluated before the breakpoint hits.

For example:

func printMarker(_ x: Int) -> Int {
    return x

struct User {
    let name: String
    let age: Int

struct Message {
    func mentionedUsersF() -> [User] {
        return [User(name: "Achilles", age: 32), User(name: "Tortoise", age: 105)]

let message = Message()

var ages: [Int]

ages = [1, 2, 3]

ages = message
    .map { $0.age }


Place a breakpoint on the `ages = message` line, and you will see that "marker" has been printed at the time of breaking. The `ages` variable is still equal to `[1, 2, 3]`, however, so it hasn't been set yet.

This also occurs if you use a computed property instead of a function.

This does not happen if:

  • Any of the function calls are moved up to the first line

  • The struct contains a non-computed property (it can be unused, as long as there is one)

  • A class is used instead of struct

  • Functions that are not members of a struct are used instead

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@adrian-prantl, granataenrico (JIRA User), what do you think?

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From the description this definitely sounds like a compiler bug.

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vedantk commented Apr 18, 2018

I looked into this recently and found that there isn't a good way to address this problem.

Any fix that enables stepping on the line "ages = message" prior to a call to map would result in inconsistent stepping behavior between Swift and C-based languages. Further, it might result in spurious extra stepping onto lines which just contain a reference to a decl.

@swift-ci swift-ci transferred this issue from apple/swift-issues Apr 25, 2022
@shahmishal shahmishal transferred this issue from apple/swift May 7, 2022
This issue was closed.
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bug Something isn't working LLDB for Swift
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