Could some types be allowed to be nested in generic functions?

Evolution Discussion
1

What are the reasons for not allowing this:

func f<T>(_ v: T) {
    struct S { // ERROR: Type 'S' cannot be nested in generic function 'f'
        let a: Int
        let b: Bool
    }
}

?

Given that the following compiles:

func g<T>(_ v: T) {
    typealias S = (
        a: Int,
        b: Bool
    )
}
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2

Nobody decided if S defined inside f<String> and inside f<Data> should be the same type or not yet

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3

In the first example, the struct is defining a completely new type with new type metadata, its own initializers and property getters/setters, and so forth. In the second, the typealias is merely a new name for an existing type; nothing new is being generated (and AFAICT, typealiases that aren't accessible outside the module aren't emitted at all in the serialized module file).

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4

What difference would it make? Ie, let's say S is the same type no matter the type parameter (because it doesn't use any type parameter), what problem can it cause? The type is only available/existing within the scope of the function afaics.

5 
func foo<T: Numeric>(_ n: T) -> Int {
  struct Bar {
    static var x: Int = 0
  }
  
  if n == 0 {
    Bar.x += 1
  }
  return Bar.x
}

print(foo(0))
print(foo(0.0))

What should get printed?

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6

Right, I didn't think about static stored properties. So it seems like S (or Bar) should not be the same type for different type parameters then, which would sort of make them generic, but static stored properties are not allowed in generic types ...

7

There's only one possible answer—yes, the type is different for each set of outer type arguments. This just hasn't been implemented yet.

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