# Reading a computed property in a method consumes self but only in some cases? **URL:** \ **Category:** Using Swift\ **Tags:** ownership, generics, properties\ **Created:** [April 24, 2024, 4:40pm UTC](https://forums.swift.org/t/reading-a-computed-property-in-a-method-consumes-self-but-only-in-some-cases/71444 "2024-04-24T16:40:31Z")\ **Posts on this page:** 6\ **Page:** 1
**Author:** ![corbeling](https://avatars.discourse-cdn.com/v4/letter/c/6f9a4e/32.png) [@corbeling](https://forums.swift.org/u/corbeling)\ **Post date:** [April 24, 2024, 4:40pm UTC](https://forums.swift.org/t/reading-a-computed-property-in-a-method-consumes-self-but-only-in-some-cases/71444/1 "2024-04-24T16:40:31Z")
Hi folks! I'm learning Swift and I've been playing around with `borrowing` and `consuming`, and I ran into something I don't quite understand. It seems that in a generic struct that has a stored property with the generic type, reading a computed property inside a `borrowing` method consumes `self`. So eg. this contrived but minimal example won't compile: ```swift struct Gleb { let base: Base var computed: Int { 666 } borrowing func doSomething() -> Int { computed } } ``` ```swift /whee/bla.swift:59:18: error: 'self' is borrowed and cannot be consumed borrowing func doSomething() -> Int { computed } ^ /whee/bla.swift:59:41: note: consumed here borrowing func doSomething() -> Int { computed } ^ ``` Changing the property to `{ borrowing get { 666 } }` doesn't help. All of these compile just fine: ```swift class ClassyGleb { let base: Base var computed: Int { 666 } borrowing func doSomething() -> Int { computed } init(base: Base) { self.base = base } } struct Baseless { var computed: Int { 666 } borrowing func doSomething() -> Int { computed } } struct Funcy { let base: Base var computed: Int { 666 } func _computed() -> Int { computed } borrowing func doSomething() -> Int { _computed() } } ``` What exactly is going on here? Why does storing a generically typed `base: Base` throw a wrench into things if you read computed properties from a `borrowing` method, but only if it's a struct? I've tried reading [SE-0377](https://github.com/apple/swift-evolution/blob/main/proposals/0377-parameter-ownership-modifiers.md) and [SE-0390](https://github.com/apple/swift-evolution/blob/main/proposals/0390-noncopyable-structs-and-enums.md), but either haven't successfully understood them or this behavior isn't covered in them. ```swift ❯ swift -version 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 ``` ---
**Author:** ![corbeling](https://avatars.discourse-cdn.com/v4/letter/c/6f9a4e/32.png) [@corbeling](https://forums.swift.org/u/corbeling)\ **Post date:** [April 26, 2024, 1:41pm UTC](https://forums.swift.org/t/reading-a-computed-property-in-a-method-consumes-self-but-only-in-some-cases/71444/2 "2024-04-26T13:41:47Z")
There's clearly something I don't understand about ownership mechanics and how they relate to computed properties, because this happens with protocols too: ```swift struct Example { protocol Proto { var count: Int { borrowing get } } func takeProto(_ p: borrowing some Proto) -> Int { p.count } } ``` This fails to compile because `'p' is borrowed and cannot be consumed` even though I again tried with an explicit `borrowing get`. This doesn't seem to fit any of the cases listed as a consuming operation in [SE-0390](https://github.com/apple/swift-evolution/blob/main/proposals/0390-noncopyable-structs-and-enums.md#consuming-operations): - assigning a value to a new let or var binding, or setting an existing variable or property to the binding - passing an argument to a consuming parameter of a function - passing an argument to an init parameter that is not explicitly borrowing - invoking a consuming method on a value, or accessing a property of the value through a consuming get or consuming set accessor - explicitly consuming a value with the consume operator - return-ing a value ---
**Author:** ![Joe\_Groff](https://sea2.discourse-cdn.com/swift/user_avatar/forums.swift.org/joe_groff/32/1929_2.png) [@Joe\_Groff](https://forums.swift.org/u/Joe_Groff)\ **Post date:** [April 26, 2024, 3:15pm UTC](https://forums.swift.org/t/reading-a-computed-property-in-a-method-consumes-self-but-only-in-some-cases/71444/3 "2024-04-26T15:15:35Z")
> [@corbeling](#): > > ```swift > struct Example { > protocol Proto { > var count: Int { borrowing get } > } > > func takeProto(_ p: borrowing some Proto) -> Int { > p.count > } > } > > ``` That looks like a bug to me. This should work. ---
**Author:** ![corbeling](https://avatars.discourse-cdn.com/v4/letter/c/6f9a4e/32.png) [@corbeling](https://forums.swift.org/u/corbeling)\ **Post date:** [April 26, 2024, 4:05pm UTC](https://forums.swift.org/t/reading-a-computed-property-in-a-method-consumes-self-but-only-in-some-cases/71444/6 "2024-04-26T16:05:07Z")
Ah, well that would make sense. Any idea if my original example should also work? Intuitively both of these cases feel like they _should_ be fine ---
**Author:** ![Joe\_Groff](https://sea2.discourse-cdn.com/swift/user_avatar/forums.swift.org/joe_groff/32/1929_2.png) [@Joe\_Groff](https://forums.swift.org/u/Joe_Groff)\ **Post date:** [April 26, 2024, 5:44pm UTC](https://forums.swift.org/t/reading-a-computed-property-in-a-method-consumes-self-but-only-in-some-cases/71444/7 "2024-04-26T17:44:36Z")
Yeah, I would expect all of your examples to compile. It compiles if the types themselves are `~Copyable`, so it should also compile if the types are Copyable but the parameter bindings have implicit copying suppressed with `borrowing`/`consuming` modifiers. ---
**Author:** ![corbeling](https://avatars.discourse-cdn.com/v4/letter/c/6f9a4e/32.png) [@corbeling](https://forums.swift.org/u/corbeling)\ **Post date:** [April 26, 2024, 5:48pm UTC](https://forums.swift.org/t/reading-a-computed-property-in-a-method-consumes-self-but-only-in-some-cases/71444/8 "2024-04-26T17:48:25Z")
> > > \### Description > > I've found two cases of values getting consumed when reading co…mputed properties that seem like they're not correct behavior based on my reading of \[SE-0377\](https://github.com/apple/swift-evolution/blob/main/proposals/0377-parameter-ownership-modifiers.md) and \[SE-0390\](https://github.com/apple/swift-evolution/blob/main/proposals/0390-noncopyable-structs-and-enums.md). > > Case 1 is a method with a \`borrowed some Proto\` parameter calling a getter on the borrowed value, and case 2 is a \`borrowing\` method in a \`struct A\\` that has a stored property \`v: T\` > > \### Reproduction > > \## Case 1: borrowed parameter with opaque type > > \`\`\`swift > struct Example { > protocol Proto { > // doesn't work with { borrowing get } either > var count: Int { get } > } > > func takeProto(\_ p: borrowing some Proto) -\> Int { > p.count > } > } > \`\`\` > > Result is > \`\`\` > error: 'p' is borrowed and cannot be consumed > func takeProto(\_ p: borrowing some Proto) -\> Int { > ^ > > note: consumed here > p.count > ^ > \`\`\` > > Interestingly using \`p: borrowing any Proto\` leads to a different error. Should I make another issue for this as the error suggests? > > \`\`\` > error: usage of no-implicit-copy value that the compiler can't verify. This is a compiler bug. Please file a bug with a small example of the bug > \`\`\` > > > \## Case 2: borrowing method in a generic struct with stored property of generic type > > \`\`\`swift > struct Broken\ { > let base: Base > var compd: Int { 666 } > borrowing func doSomething() -\> Int { compd } > } > \`\`\` > > \`\`\` > error: 'self' is borrowed and cannot be consumed > borrowing func doSomething() -\> Int { compd } > ^ > note: consumed here > borrowing func doSomething() -\> Int { compd } > ^ > \`\`\` > > > \### Expected behavior > > \## Case 1 > > Expected no error > > > \## Case 2 > > Expected no error, like in the following cases that compile fine: > > > \`\`\`swift > class Class\ { > let base: Base > var computed: Int { 666 } > > borrowing func doSomething() -\> Int { computed } > > init(base: Base) { > self.base = base > } > } > > struct NoProp\ { > var computed: Int { 666 } > borrowing func doSomething() -\> Int { computed } > } > > struct NotGeneric { > var computed: Int { 666 } > borrowing func doSomething() -\> Int { computed } > } > \`\`\` > > > \### 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 > > \[Forum post\](https://forums.swift.org/t/reading-a-computed-property-in-a-method-consumes-self-but-only-in-some-cases/71444))