Compiler fails to prevent data race in generic function

[rayx]

Description

Compiler produces diagnostics for the following (as expected):

func test() {
    var result = 0
    Task {
        result = 0
    }
    print(result) // note: access can happen concurrently
}

but fails to produces diagnostics for similar generic code:

func test<T: Sendable>(t: T) {
    var result: T = t
    Task {
        result = t
    }
    print(result)
}

test(t: 1)

Reproduction

See above

Expected behavior

The generic function should fails to compile too.

Environment

I used x86-64 swiftc nightly on godbolt.

Additional information

No response

[slavapestov]

If you add an AnyObject constraint, it also diagnoses:

func test<T: Sendable & AnyObject>(t: T) {
    var result: T = t
    Task {
        result = t
    }
    print(result)
}

final class C: Sendable {}

test(t: C())

[rayx]

Below are more experiment results. It covers two scenarios (non-isolated function parameter vs local variable) and tries different constraints in each scenario. The result shows that Sendable conformance somehow confuses compiler.

1) Scenario 1: parameter of non-isolated function

The test is invalid and should always produce diagnostics no matter if T has constraint or what constraint it has.

func test<T>(obj: T) {
    var result = obj
    Task {
        result = obj
    }
}

Actual test result:

T's Constraint	Producing Diagnostics?
No constraint	Yes
Sendable	No
AnyObject	Yes
Sendable & AnyObject	No

2) Scenario 2: local variable

The test is invalid and should always produce diagnostics no matter if T has additional constraints

protocol Initializable {
    init()
}

func test<T: Initializable & SendableMetatype>(type: T.Type) {
    var result = type.init()
    Task {
        result = type.init()
    }
    print(result)
}

Actual test result:

Additional Constraints	Producing Diagnostics?
Sendable	No
AnyObject	Yes
Sendable & AnyObject	Yes

[slavapestov]

Same problem with a non-generic function that uses an address-only existential:

func test() {
    var result: Any = 0
    Task {
        result = 0
    }
    print(result)
}

[gottesmm]

Fix: #85164