# Is this a function builder bug or is my builder incorrect?

**URL:** <https://forums.swift.org/t/is-this-a-function-builder-bug-or-is-my-builder-incorrect/35144>\
**Category:** Using Swift\
**Created:** [April 6, 2020, 5:08pm UTC](https://forums.swift.org/t/is-this-a-function-builder-bug-or-is-my-builder-incorrect/35144 "2020-04-06T17:08:24Z")\
**Posts on this page:** 3\
**Page:** 1

<div class="post-metadata">

**Author:** ![DevAndArtist](https://sea2.discourse-cdn.com/swift/user_avatar/forums.swift.org/devandartist/32/287_2.png) [@DevAndArtist](https://forums.swift.org/u/DevAndArtist)\
**Post date:** [April 6, 2020, 5:08pm UTC](https://forums.swift.org/t/is-this-a-function-builder-bug-or-is-my-builder-incorrect/35144/1 "2020-04-06T17:08:24Z")

</div>

cc @Douglas_Gregor, I'm 90% sure the single expression worked in Xcode 11.3.x if we added `wrapIntoAnyP` manually as it previously had no support for `buildExpression`. Now in Xcode 11.4 non of the single expressions seem to work. The compiler does not want to pick up the expression convert it into `AnyP` and then build the block from it.

```swift
struct AnyP<T>: P {
  typealias C = T
  let value: Any
}

protocol P {
  associatedtype C
}

extension P {
  func wrapIntoAnyP() -> AnyP<C> {
    AnyP(value: self)
  }
}

extension String: P {
  typealias C = Int
}
extension Bool: P {
  typealias C = Int
}

extension Optional: P where Wrapped: P {
  typealias C = Wrapped.C
}

@_functionBuilder
enum Builder {
  static func buildExpression<T, C>(
    _ content: T
  ) -> AnyP<C> where T: P, T.C == C {
    content.wrapIntoAnyP()
  }

  static func buildIf<T, C>(
    _ content: T?
  ) -> AnyP<C> where T: P, T.C == C {
    content.wrapIntoAnyP()
  }

  static func buildBlock<C>(
    _ components: AnyP<C>...
  ) -> [AnyP<C>] {
    components
  }
}

func foo<T>(@Builder test: () -> [T]) where T: P {}

// OKAY
foo {
  true
  "swift"
}

foo { // error: Generic parameter 'T' could not be inferred
  true // error: Cannot convert value of type 'Bool' to closure result type '[T]'
}

```

---

<div class="post-metadata">

**Author:** ![DevAndArtist](https://sea2.discourse-cdn.com/swift/user_avatar/forums.swift.org/devandartist/32/287_2.png) [@DevAndArtist](https://forums.swift.org/u/DevAndArtist)\
**Post date:** [April 6, 2020, 5:18pm UTC](https://forums.swift.org/t/is-this-a-function-builder-bug-or-is-my-builder-incorrect/35144/2 "2020-04-06T17:18:48Z")

</div>

Also the `if` expression seems not to work either. 🤔

```swift
// error: Unable to infer complex closure return type; add explicit type to disambiguate
foo {
  if true {
    "swift"
  }
  "swift"
}

```

I assume the compiler to pick up the expression, build `AnyP`, wrap it into an optional, wrap that into `AnyP`, pick up the last string, wrap it into `AnyP` and finally build the block from several `AnyP`'s.

---

<div class="post-metadata">

**Author:** ![typesanitizer](https://sea2.discourse-cdn.com/swift/user_avatar/forums.swift.org/typesanitizer/32/8661_2.png) [@typesanitizer](https://forums.swift.org/u/typesanitizer)\
**Post date:** [April 6, 2020, 11:30pm UTC](https://forums.swift.org/t/is-this-a-function-builder-bug-or-is-my-builder-incorrect/35144/3 "2020-04-06T23:30:58Z")

</div>

Please file a bug report on [bugs.swift.org](http://bugs.swift.org) if you suspect there is a bug here (and any information you have about past compiler versions where it was known to work). It's ok if you are not sure whether it is supposed to work or not.