# `@_lifetime(copy self)` on `consuming async` protocol witness rejected when protocol method is `nonisolated(nonsending)`

[FranzBusch]

Description

When declaring a protocol method with a @_lifetime(copy self) annotation and making it nonisolated(nonsending) a protocol witness is being rejected. The error references a protocol witness thunk (...FTW symbol), suggesting that there is a thunk generated that doesn't correctly apply the lifetime attribute.

error: lifetime-dependent value escapes its scope
   |
40 |     @_lifetime(copy self)
41 |     consuming func send() async throws -> BaseWriter {
   |                    |- error: lifetime-dependent value escapes its scope
   |                    |- note: it depends on the lifetime of an argument of '$s5Repro10BaseSenderVAA0C0A2aDP4send1WQzyYaKFTW'
   |                    `- note: this use causes the lifetime-dependent value to escape
42 |         return BaseWriter()
43 |     }

The mangled symbol $s5Repro10BaseSenderVAA0C0A2aDP4send1WQzyYaKFTW decodes to:

protocol witness for Repro.Sender.send() async throws -> A.W
in conformance Repro.BaseSender : Repro.Sender in Repro

Reproduction

Running the following code with .enableExperimentalFeature("Lifetimes")

protocol Writer: ~Copyable, ~Escapable {}

protocol Sender<W>: ~Copyable, ~Escapable {
    associatedtype W: Writer, ~Copyable, ~Escapable

    @_lifetime(copy self)
    nonisolated(nonsending) consuming func send() async throws -> W
}

struct BaseWriter: Writer, ~Copyable, ~Escapable {
    @_lifetime(immortal)
    init() {}
}

struct BaseSender: Sender, ~Copyable, ~Escapable {
    typealias W = BaseWriter

    @_lifetime(immortal)
    init() {}

    @_lifetime(copy self)
    nonisolated(nonsending) consuming func send() async throws -> BaseWriter {
        return BaseWriter()
    }
}

Expected behavior

The code should compile

Environment

swift-driver version: 1.166 Apple Swift version 6.4 (swiftlang-6.4.0.19.4 clang-2100.3.19.4)

Additional information

No response