[swift-build-dev] error: ambiguous use of operator

[Ryan Lovelett]

I've got a package that I'm working on that is running into a weird
error. When I run `swift build` it is failing with:

```
/private/tmp/langserver-swift/Sources/JSONRPC/Types/Request.swift:54:46:
error: ambiguous use of operator '<|'
        let dMethod: Decoded<String> = (json <| "method")
                                             ^
Argo.<|:14:13: note: found this candidate
public func <|<A : Decodable where A == A.DecodedType>(json: Argo.JSON,
key: String) -> Argo.Decoded<A>
            ^
Argo.<|:14:13: note: found this candidate
public func <|<A : Decodable where A == A.DecodedType>(json: Argo.JSON,
key: String) -> Argo.Decoded<A>
```

The problem is that when I tell SwiftPM to build an Xcode project for me
and build in Xcode then it compiles just fine. You can experience this
behavior for yourself in this repository:
https://github.com/RLovelett/langserver-swift

Is this a bug in my Package.swift or with SwiftPM?