[SR-15200] Returning `eventLoop.future<T?, U?>(nil, nil)` from `ELF.flatMap(_:)` causes memory lifetime verifier failure

[gwynne]

Previous ID	SR-15200
Radar	rdar://problem/83190880
Original Reporter	@gwynne
Type	Bug
Status	Resolved
Resolution	Done

Environment

Swift version 5.5-dev (LLVM 27ee1ffec3b6967, Swift c7dd41bf2c475d0)
Target: x86_64-unknown-linux-gnu
Linux ubuntu 5.11.0-1016-aws #​17~20.04.1-Ubuntu SMP Thu Aug 12 05:39:36 UTC 2021 x86_64 x86_64 x86_64 GNU/Linux

Additional Detail from JIRA

Votes	0
Component/s	Compiler
Labels	Bug, CompilerCrash
Assignee	@eeckstein
Priority	Medium

md5: ce5a1a391a7f060cb7e032d966aaa304

Issue Description:

Partial redacted compiler output:

SIL memory lifetime failure in @$s3App13FooControllerV9createFoo3req7NIOCore15EventLoopFutureCy8NIOHTTP118HTTPResponseStatusOG5Vapor7RequestC_tKFAHyAA20ModelAAExternalInputCSg_AA012ModelBBBBBBBrS0CSgtGAUcfU5_: memory is initialized, but shouldn't
memory location:   %26 = alloc_stack $(Optional<ModelBBBBBBBExternalInput>, Optional<ModelAAExternalInput>) // users: %40, %39, %27
at instruction:   dealloc_stack %26 : $*(Optional<ModelBBBBBBBExternalInput>, Optional<ModelAAExternalInput>) // id: %40
<snip>
1.  Swift version 5.5-dev (LLVM 27ee1ffec3b6967, Swift c7dd41bf2c475d0)
2.
3.  Contents of /tmp/TemporaryDirectory.5Hlpso/sources-1:
<snip>
4.  While evaluating request ASTLoweringRequest(Lowering AST to SIL for file "/home/ubuntu/foo/Sources/App/Controllers/FooController.swift")

[ktoso]

@swift-ci create

[typesanitizer]

Gwynne, could you share a code example?

[eeckstein]

theindigamer (JIRA User) I talked with Gwynne offline. I don't need a test case. I already know what the problem is

[gwynne]

theindigamer (JIRA User) For what it's worth, I wasn't able to find a repro case any smaller than the actual code where I ran into the issue (a huge Vapor-based service with 1500 source files) - far as I could tell it had something to do with this construct:

func blah(_ req: Request) -> EventLoopFuture<...> {
    return doStuffReturningOptional().flatMap { (obj1: ModelA?) -> EventLoopFuture<(ModelA?, ModelB?)> in
        if obj != nil {
            return req.eventLoop.makeSucceededFuture((nil, nil)) // <-- this (nil, nil) tuple is the proximate culprit
        }
        return doSomeStuffReturningAnotherObject().flatMap { obj2 in
            return doSomeMoreStuff().map { (obj1, obj2) }
        }
    }
}

I couldn't figure out a repro despite having it narrowed down this far, but fortunately Erik figured out the problem (whatever it is! 😃) from the details I gave offline.

[eeckstein]

This is already fixed on main by #38128