cc @DougGregor

Comment by Edison Santiago (JIRA)

Just tried to compile it with the new Xcode 9.3 Beta and still got the same error =/ Am I doing something wrong? Wasn't that supposed to work this way?

The problem here was the compiler was "accidentally" not implying conformances, that is, writing a conformance to a child protocol wouldn't automatically create the conformance to the parent. As of #15268 (which is included in the 2018-04-04 development snapshot), the compiler gives even better errors:

Conditional conformance of type 'UseResource<T>' to protocol 'UseSpecializedResourceProtocol' does not imply conformance to inherited protocol 'UseResourceProtocol'

It also includes some (currently slightly syntactically incorrect, but fixing them now) fixits that offer two options for an appropriate conformance.

Additionally, the extension UseResourceProtocol declaring foo is used: every protocol conformance needs to provide all the methods and properties that the protocol wants, and just the plain extension UseResource: UseResourceProtocol where T: ResourceProtocol (let's call this extension A) doesn't have a foo that applies (the one from extension UseSpecializedResourceProtocol where T: SpecializedResourceProtocol doesn't work, because the UseResource in extension A isn't known to conform to UseSpecializedResourceProtocol, as that conformance needs T: SpecializedResourceProtocol, but we only have T: ResourceProtocol). The extension UseResourceProtocol is behaving the same as

extension UseResource: UseResourceProtocol where T: ResourceProtocol {
    static func foo(t: T) -> String { 
        return "foo"
    }
}

UseResource<MyModel>.foo(t: MyModel()) calls the UseSpecializedResourceProtocol version of foo, because that's the most specialised foo that works (MyModel conforms to SpecializedResourceProtocol, so UseResource conforms to UseSpecializedResourceProtocol). If we do the same call with a type that only conforms to ResourceProtocol, it prints "foo" as expected:

struct MyModel2: MainProtocol, ResourceProtocol {}
print(UseResource<MyModel2>.foo(t: MyModel2())) 

As the diagnostics have dramatically improved, I'm going to tentatively closed this as resolved. What do you think edisonlsm (JIRA User)?

Comment by Edison Santiago (JIRA)

Hello @huonw

Just downloaded the 2018-04-04 toolchain and the new compiler error message made it much better And it worked with the Xcode 9.3 default Swift, yay 😃

extension UseResource: UseResourceProtocol where T: SpecializedResourceProtocol {}

Since I have a lot of protocols like `UseResourceProtocol` in my code here (I only used one to illustrate the problem, but I have more), I need for the conformance to be on the extension and making it conform to `UseResourceProtocol` and `UseSpecializedResourceProtocol` solved it for me!

I assumed that the conformance would be implied to the parent protocol and that's what caused my entire problem here. Is this a bug at this moment or is this the intended behavior of the language?

It was a good assumption to make, and it was incorrect of the compiler to be so confusing when people make that assumption.

Having the implied conformance is how it works for non-conditional ones, and was actually what the proposal for conditional conformances originally said. However, we realised that this behaviour may be a surprising and suboptimal in some cases, so we recently amended that proposal to change the behaviour. If you're interested in the reasoning, that pull request and the discussion thread go into some detail, and feel free to ask any questions here or on the forum.

For your specific code, one could phrase the reasoning as: what if UseResource: UseResourceProtocol was actually meant to apply for T: ResourceProtocol, not T: SpecializedResourceProtocol. If the conformance was implied, you wouldn't notice that you're accidentally getting the second bound instead of the first.