Missing `sending` for subscript sending result

[stzn]

Description

When the sending is applied to the return type in a subscript and SIL is emitted, the sending disappears from the output.

Here's an example to demonstrate this behavior:

class NonSendableKlass {}

struct S {
    subscript(_: sending NonSendableKlass) -> sending NonSendableKlass { NonSendableKlass() }
}

The SIL output shows:

sil_stage raw

...
struct S {
  subscript(_: sending NonSendableKlass) -> NonSendableKlass { get }
  init()
}

...

// S.subscript.getter
// Isolation: unspecified
sil hidden [ossa] @$s4main1SVyAA16NonSendableKlassCAEncig : $@convention(method) (@sil_sending @owned NonSendableKlass, S) -> @owned NonSendableKlass {

Note that while the parameter retains its sending in the SIL output, the return type's sending is missing.

Reproduction

class NonSendableKlass {}

struct S {
    subscript(_: sending NonSendableKlass) -> sending NonSendableKlass { NonSendableKlass() }
}

Expected behavior

sending and @sil_sending are outputted.

Environment

swift-driver version: 1.115.1 Apple Swift version 6.0.3 (swiftlang-6.0.3.1.10 clang-1600.0.30.1)
Target: arm64-apple-macosx15.0

Additional information

No response