Wrong Sendable closure diagnostics with implicitly unwrapped optional

[groue]

Description

Hello,

The Swift compiler 5.10 that ships with Xcode 15.3 (15E204a) emits spurious warnings regarding sendable closures, in the specific case of implicitly unwrapped optionals.

Reproduction

In the following code, the compiler emit false positive warnings. They are all related to implicitly unwrapped optionals.

Converting non-sendable function value to '@Sendable () -> Void' may introduce data races

struct ClosureHolder {
    var closure: @Sendable () -> Void = { }
    var closureOpt: (@Sendable () -> Void)?
    var closureIUO: (@Sendable () -> Void)!
}

func f() {
    var holder: ClosureHolder = ClosureHolder()
    holder.closure = { }        // OK
    holder.closureOpt = { }     // OK
    holder.closureIUO = { }     // ⚠️ WARNING

    var holderOpt: ClosureHolder?
    holderOpt?.closure = { }    // OK
    holderOpt?.closureOpt = { } // OK
    holderOpt?.closureIUO = { } // ⚠️ WARNING

    var holderIUO: ClosureHolder!
    holderIUO.closure = { }     // ⚠️ WARNING
    holderIUO.closureOpt = { }  // ⚠️ WARNING
    holderIUO.closureIUO = { }  // ⚠️ WARNING
}

Expected behavior

The compiler emits no warning.

Environment

swift-driver version: 1.90.11.1 Apple Swift version 5.10 (swiftlang-5.10.0.13 clang-1500.3.9.4) Target: arm64-apple-macosx14.0

Additional information

No response

[groue]

Reproducible with both swift-DEVELOPMENT-SNAPSHOT-2024-03-29-a and swift-6.0-DEVELOPMENT-SNAPSHOT-2024-03-26-a.

[groue]

The bug is still present in Xcode 16.0 beta (16A5171c).

[groue]

The bug is still present in Xcode 16.0 beta 3 (16A5202i).