无法将协议传递给受约束的通用类型

Question

If you have a function which has a generic type that is constrained to a protocol you cannot pass a value of the type of the protocol to the function:

protocol P {}

extension Int: P {}

func testP<T: P>(value: T) {}

// passing Int
let val = 0
testP(val)

// passing Int as P (protocol)
let valAsP: P = 0
testP(valAsP) // error: cannot invoke 'testP' with an argument list of type '(P)'

If I change the protocol to a class (and using inheritance instead) the same function would be executed in both cases.

In addition "The Swift Programming Language" book says that

The colon in the declaration means “…of type…,”

If this is also true for generic constraints I would argue it is a bug.

So should this be considered a bug? Or is there any case where it is useful to have this behavior?

Answer 1

Yes, per the spec on the definition of Types section:

A named type is a type that can be given a particular name when it is defined. Named types include classes, structures, enumerations, and protocols .

protocols are a valid, named type.

Also supported by the fact that if you remove the P constraint from testP , the code compiles.

You can resolve your scenario by changing the signature of testP to

func testP(value: P) {}

which should work for most scenarios.

Incidentally, your scenario is possible in C# where interfaces are similar to protocols in Swift

I have created issue SR-1324