Why aren't negative generic parameters possible in Swift?

Explain what the system is supposed to do when you have two of those

struct Foo <T where T != Int> {}

struct Foo <T where T != String> {}

and you say Foo<Something>;

Edit: The bonus questions are "So how do you instantiate a template with no type information (since T != Int doesn't actually tell you what type T is)?"

Perhaps you're being a little unfair. What the system can do is complain of the ambiguity, the same way it currently does when it can't decide which speciailization to use.

Bonus answer: In something simpler like:

struct Foo T> { … }

you also have "no type information". You instantiate it using the type that it matches when it matches.

What prize do I get for answering the bonus question?

Suppose we have a type which takes two generic types A, B and uses them for subscripts. This code would be confusing if A and B were the same type so specifying that A != B would be good.

struct Foo <A, B>

{

subscript (index : A) -> C { }

subscript (index : B) -> D { }

}

Also, this could allow for value-type-only specialization. For example:

protocol Foo
{
   
}

extension Foo where Self != AnyObject
{

}

The following should answer your question:

protocol X
{

}

extension X
{
    func foo()
    {
        print("I'm an X foo")
    }
}

protocol Y
{

}

extension Y
{
    func foo()
    {
        print("I'm a Y foo")
    }
}

protocol Z
{
    func foo()
}

struct Foo: Z, X, Y
{

}

For your conveniece, is the playground output:

Playground execution failed: /var/folders/rv/mwcdgk492x7g23_0byfwyjtw0000gn/T/./lldb/1444/playground261.swift:44:8: error: type 'Foo' does not conform to protocol 'Z'
struct Foo: Z, X, Y
       ^
/var/folders/rv/mwcdgk492x7g23_0byfwyjtw0000gn/T/./lldb/1444/playground261.swift:41:10: note: multiple matching functions named 'foo()' with type '() -> ()'
    func foo()
         ^
/var/folders/rv/mwcdgk492x7g23_0byfwyjtw0000gn/T/./lldb/1444/playground261.swift:33:10: note: candidate exactly matches
    func foo()
         ^
/var/folders/rv/mwcdgk492x7g23_0byfwyjtw0000gn/T/./lldb/1444/playground261.swift:20:10: note: candidate exactly matches
    func foo()         ^