Swift ambiguous reference to member '=='

Question

Is this a bug in Swift 3 compiler? It's not ambiguous, it's `==` and two `String`s. It says: ``` error: ambiguous reference to member '==' let strs = things.filter { return $0.id == "1" } .map { t in ^~ ``` For this example code: ``` class Thing { var id: String = "" } let things = [Thing]() let x = 1 let strs = things.filter { return $0.id == "1" } .map { t in if x == 1 { return "a" } else { return "b" } } ```

Accepted Answer

It's a misleading error message – the ambiguity actually lies with the closure expression you're passing to `map(_:)`, as Swift cannot infer the return type of a multi-line closure without any external context.

So you could make the closure a single line using the ternary conditional operator:

```
let strs = things.filter { $0.id == "1"} .map { _ in
    (x == 1) ? "a" : "b"
}
```

Or simply supply the compiler some explicit type information about what `map(_:)` is returning:

```
let strs = things.filter { $0.id == "1"} .map { _ -> String in
```

```
let strs : [String] = things.filter { $0.id == "1"} .map { _ in
```

Swift ambiguous reference to member '=='

Solution

Problem