v42.00 - Game + UEFN immediate crash

[OKomili]

Summary

We have been unable to launch a session of our game in Fortnite, nor are we able to launch the UEFN editor of our project. There is an immediate crash, and because we don’t have the debug symbols, we’re unable to see the stack trace. We’re trying to investigate as best we can from the little details we see in the logs, but aren’t able to figure things out.

We’d appreciate some help ASAP. We just launched our game on Monday and were getting some momentum, I don’t want to lose that now.

Please select what you are reporting on:

Unreal Editor for Fortnite

What Type of Bug are you experiencing?

Stability

Steps to Reproduce

Open the editor, or launch a session in Fortnite

Expected Result

It works

Observed Result

It crashes

Platform(s)

PC

Island Code

9621-3035-2804

Additional Notes

I used Claude to try and analyze the logs, here is a summary it has provided. I’m not sure if this is helpful.

UEFN 42.00 (CL-56878558): Verse compiler ICE on project open — IRGenerator.cpp:1889

Project opened fine on 41.30 (CL-56430492). After the 42.00 update it crashes every time during OpenProject_BuildVerse, before the editor is usable.

Ensure condition failed: bConstrained
[File: Engine\Source\Runtime\VerseCompiler\Private\uLang\SemanticAnalyzer\IRGenerator.cpp] [Line: 1889]
IrArgument->IrGetResultType() must be a subtype of IrInstCalleeType->GetParamsType()
The ensure fires once, then the editor takes an unhandled 0x80000003. No Verse file, line, or symbol is reported, so there is no way to locate the offending construct from the project side — that’s the main blocker. ~229 Verse files. Semantic analysis appears to pass; the failure is in IR generation.

Ruled out from our side: nothing we call was removed in 42.00 (diffed the 41.30 and 42.00 digests; the API is essentially additive). We also removed all our signalable/awaitable implementations — those interfaces became in 42.00 — with no change to the crash.

Ask: symbolicate the attached dump, or have the compiler report a file/line on this ensure path.

[OKomili]

Logs: https://drive.google.com/file/d/1gN5Noa3tsMJqxmwEtyPPdTBjioiIcQQ-/view?usp=drive_link

[Flak]

Thank you, the team is checking into this.

[Jimbohalo10]

hi all,

Found what could be the change for UE6-main in source code, hopefully will help Epic Support but wont help you.

The source code just says that a comparison inside square brackets was being generated at the line with Error .

No Verse information sorry.

Insert a coercion on the right-hand side of an assignment

[OKomili]

FYI I’ve put the island in an Unlisted state so as to not confuse players as to why the game won’t load and risk losing them in the future.

[Bug-Reporter]

The status of FORT-1142794 changed to ‘Needs Triage’. We are routing this to the appropriate team for investigation.

[OKomili]

@Flak

EDIT: Note that I have not confirmed that this is the ONLY crash, but it is one of them. I’m trying to apply a fix to my project now to see if I can get it to launch.

ORIGINAL POST BELOW
I’ve been working with Claude for the past 8 hours and have managed to identify exactly what is causing the crash, and have created a minimum proof of concept to reproduce the crash.

Create an empty project, create a verse file, put this code in, and compile. The editor crashes immediately and the project can no longer be opened.

repro_metric<public>       := type {_X:int where _X >= 0}
repro_stock_metric<public> := type {_X:int where 100000 <= _X, _X < 1000000}

repro_has_metric<public> := interface:
    Metric<public>:repro_metric

repro_stock<public> := class<abstract>(repro_has_metric):
    Metric<override>:repro_stock_metric      # narrowed

repro_wheat<public> := class(repro_stock):
    Metric<override>:repro_stock_metric = 100001

repro_manager<public> := class:
    AllStock:[]repro_stock = array:
        repro_wheat{}

    block:
        for (Stock : AllStock, set MetricStockMap[Stock.Metric] = Stock) {}   # crash

    var<private> MetricStockMap:[repro_stock_metric]repro_stock = map{}

[OKomili]

Me again. While I was able to find a workaround solution to get the project to compile, launch session, complete a memory test, etc… I’m now having a problem trying to publish a new build. During the Publish step, I get a Verse Validation error with a very generic and unhelpful error. There is no indication on what’s wrong. Note that the Memory Test works fine, and during the mem test this window pops up showing the Cook was successful.

I don’t know if this is related, or an entirely separate issue.