False positive in isolation checking subscript isolated to an argument

[nickolas-pohilets]

Description

Attached examples demonstrates a valid code which produces unexpected compilation error

Reproduction

struct WritableActorKeyPath<Root: Actor, Value>: Sendable {
    var getter: @Sendable (isolated Root) -> Value
    var setter: @Sendable (isolated Root, Value) -> Void

    subscript(_ root: isolated Root) -> Value {
        get { getter(root) } // error: call to actor-isolated function in a synchronous actor-isolated context
        nonmutating set { setter(root, newValue) } // error: call to actor-isolated function in a synchronous actor-isolated context
    }
}

Expected behavior

Example should compile without errors

Environment

$ xcrun swiftc -version
swift-driver version: 1.115 Apple Swift version 6.0.2 (swiftlang-6.0.2.1.2 clang-1600.0.26.4)
Target: arm64-apple-macosx15.0

Additional information

Note that issue does not reproduce for plain functions, only for subscripts:

func testGet<Root: Actor, Value>(_ root: isolated Root, getter: (isolated Root) -> Value) -> Value {
    return getter(root) // ok
}

func testSet<Root: Actor, Value>(_ root: isolated Root, setter: (isolated Root, Value) -> Void, newValue: Value) {
    setter(root, newValue) // ok
}

No response

[nickolas-pohilets]

Looks like isIsolated flag is not copied here -

swift/lib/AST/Decl.cpp

Line 11116 in 82729df

if (subscriptParam->isCallerIsolated())