1

I've encountered an interesting issue related to protocol associated types and their conformance to AdditiveArithmetic, specifically when working with SIMD3<Float>. Despite SIMD3<Float> conforming to AdditiveArithmetic, the Swift compiler fails to recognize this conformance in the context of a protocol's associated type constraint.

Consider the following code snippet:

protocol P { 
    associatedtype A: AdditiveArithmetic 
}

struct MyFloat: P { 
    typealias A = Float 
    // Okay
}

struct MyVec3: P { 
    typealias A = SIMD3<Float> 
    // Compiler Error: Type `MyVec3` does not conform to protocol 'P'
    // Compiler Note: Possibly intended match 'MyVec3.A' (aka 'SIMD3<Float>') does not conform to 'AdditiveArithmetic'
}

// Adding this extension resolves the compiler error
extension SIMD3<Float>: AdditiveArithmetic {}
2

It isn't:

func foo(_ x: any AdditiveArithmetic) {}
foo(SIMD3<Float>()) // 🛑 Argument type 'SIMD3<Float>' does not conform to expected type 'AdditiveArithmetic'