When generic parameter could not be inferred

Using Swift
, ,
7

Extending your gist slightly below:

protocol Theme {
	associatedtype Kind
}

struct Theme1: Theme {
	typealias Kind = String
	let kind: Kind
}

let theme1 = Theme1(kind: "abc")

struct Theme2: Theme {
	typealias Kind = Int
	let kind: Kind
}

let theme2 = Theme2(kind: 999) 

#if SOME_DEFINE
typealias TheTheme = Theme1
let myTheme = theme1
#else
typealias TheTheme = Theme2
let myTheme = theme2
#end

func current() -> TheTheme {
	return myTheme
}

Yeah, so you do definitely need to perform type-erasure here. Your type erasure can be a lot more simple though:

protocol Theme {
    associatedtype Kind
    var value: Kind { get }
}

struct ThemeS: Theme {
    typealias Kind = String
    let value: Kind
}

struct ThemeI: Theme {
    typealias Kind = Int
    let value: Kind
}

class AnyTheme<WrappedKind>: Theme {
    typealias Kind = WrappedKind

    let value: Kind

    init<T: Theme>(_ wrapped: T) where T.Kind == WrappedKind {
        self.value = wrapped.value
    }
}

let current_theme1 = ThemeS(value: "Hello")

func currentTheme() -> AnyTheme<String> {
    return AnyTheme(current_theme1)
}

let t = currentTheme()
print(t)