Why are where clauses only valid on functions with generic parameters?

Question

It seems absurd that this method signature does not compile in Swift 4:

class Bar<ValueType> { 
    func version() throws -> String where ValueType == [String: Any] { ... }
}   

(Error: where clause cannot be attached to a non-generic declaration)
but this compiles fine:

class Bar<ValueType> { 
   func version<T>(_ foo: T? = nil) throws -> String where ValueType == [String: Any] { ... }  
} 

Anyone have insight as to why this is the case?

Answer 1

Because ValueType has nothing to do with this method (in the first example). It would be wrong to put such a method in a type (class/struct/enum), since it's not really a true member of that type. It's conditionally a member of that type, depending on the truth value of the where clause.

To achieve this, you would want to put this method in an extension of your type, with the where clause you want. E.g.

extension YourType where ValueType == [String: Any] {
    func version() throws -> String { ... }
}