[swift-users] How to use a protocol to infer a generic type with constraints

[Satoshi Nakagawa via swift-users]

Hi,

I got a build error "Generic parameter 'T' could not be inferred" for the
following code.

Can anyone explain why we can't use a protocol to infer the generic type T?

class Emitter {
    func emit<T: Emittable>() -> T {
        ...
    }
}

protocol Emittable {}
protocol Subemittable: Emitable {}

class ConcreteEmittable: Subemittable {}

func testCode() {
    let emitter = Emitter()

    // Error: Generic parameter 'T' could not be inferred
    let _: Emittable = emitter.emit()

    // Error: Generic parameter 'T' could not be inferred
    let _: Subemittable = emitter.emit()

    // This works
    let _: ConcreteEmittable = emitter.emit()
}

Thanks,
Satoshi

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