Can't send into Mutex

[KeithBauerANZ]

Description

I think this is a bug; if it's not a bug it's certainly a diagnostics QoL issue.

Reproduction

import Synchronization

open class NonSendable {}

class C {
    let m = Mutex(NonSendable?.none)

    func f(_ o: sending NonSendable) {
        m.withLock { $0 = o }
    }
}

swiftc -swift-version 6 SendIntoMutex.swift
SendIntoMutex.swift:9:29: error: 'inout sending' parameter '$0' cannot be task-isolated at end of function
 7 | 
 8 |     func f(_ o: sending NonSendable) {
 9 |         m.withLock { $0 = o }
   |                             |- error: 'inout sending' parameter '$0' cannot be task-isolated at end of function
   |                             `- note: task-isolated '$0' risks causing races in between task-isolated uses and caller uses since caller assumes value is not actor isolated
10 |     }
11 | }

Expected behavior

I think this code should compile. If it shouldn't compile, it should clearly state what's wrong with it.

Environment

swift-driver version: 1.120.5 Apple Swift version 6.1.2 (swiftlang-6.1.2.1.2 clang-1700.0.13.5)
Target: arm64-apple-macosx15.0

Additional information

No response