IRGen crash: async function with typed throws and nested error type under generic

[coenttb]

Description

The Swift compiler crashes with signal 11 during IR generation when an async function uses typed throws with a nested error type under any generic type.

Environment

• Swift version: 6.2.3 (swiftlang-6.2.3.3.21 clang-1700.6.3.2)
• Target: arm64-apple-macosx26.0
• Crash location: swift::irgen::emitAsyncReturn

Minimal Reproduction (4 lines)

public enum Box<T> {
    public enum Error: Swift.Error { case fail }
    public static func go() async throws(Error) {}  // CRASHES
}

Full reproduction repository: https://github.com/coenttb/swift-issue-irgen-async-typed-throws-noncopyable

To Reproduce

echo 'public enum Box<T> { public enum Error: Swift.Error { case fail }; public static func go() async throws(Error) {} }' > /tmp/crash.swift
swiftc -parse-as-library -emit-ir /tmp/crash.swift

Stack Trace

error: compile command failed due to signal 11 (use -v to see invocation)
Stack dump:
0.  Program arguments: swift-frontend -frontend -emit-ir ...
1.  Apple Swift version 6.2.3 (swiftlang-6.2.3.3.21 clang-1700.6.3.2)
2.  Compiling with effective version 5.10
3.  While evaluating request IRGenRequest(IR Generation for file "...")
4.  While emitting IR SIL function "@$s6test103BoxO2goyyYaAC5ErrorOyx_GYKFZ".
 for 'go()' (at /tmp/crash.swift:1:...)
...
4  swift-frontend  swift::irgen::emitAsyncReturn(...) + 904

Conditions Required

All three conditions must be present:

1. Generic type - any generic parameter (e.g., Box<T>)
2. Nested error type - error type defined inside the generic
3. Async + typed throws - async throws(NestedError)

Verified Test Results

Test	Description	Result
Sync function	throws(Error) without async	✅ Compiles
Untyped throws	async throws (not typed)	✅ Compiles
Non-generic	Box without generic parameter	✅ Compiles
Top-level error	throws(TopLevelError)	✅ Compiles
Nested return only	Nested return type, top-level error	✅ Compiles
Typealias workaround	typealias Error = HoistedError	✅ Compiles
With ~Copyable	Box<T: ~Copyable>	❌ Crashes
Minimal generic	Box<T> with nested error	❌ Crashes

Key finding: The crash is specifically triggered by nested error type in typed throws under any generic. ~Copyable is not required.

Workaround

Hoist error types to top-level and re-export via typealiases:

public enum BoxError: Swift.Error { case fail }

public enum Box<T> {
    public typealias Error = BoxError
    public static func go() async throws(Error) {}  // Works
}

Related Issues

• typed-throws with nested generic type crashes compiler #77297 - Typed throws with nested generic type crashes compiler
• Async init With Typed Throw Crashes In Generic struct, class, and actor #83011 - Async typed throws in generic context crashes compiler

Impact

This blocks adoption of typed throws in libraries using generics with nested error types and async APIs - a common pattern in I/O, networking, and resource management libraries.