Open Radar

 
Summary:
A struct/class/enum can have type parameters, and its methods can constrain those type parameters, but the constraints are ignored.


Steps to Reproduce:
1. Compile this:
struct S<T> {
	func f<U where T: Hashable>() -> Set<T> {
		return Set()
	}
}


Expected Results:
I expected this to compile. Alternatively, it would be nice to have a helpful error message explaining why this isn’t supported, even if it’s just “methods cannot constrain the receiving type’s parameters because that’s unimplemented.”


Actual Results:
./boom.swift:4:50: error: type 'T' does not conform to protocol 'Hashable'


Regression:
N/A


Notes:
Note that it makes no difference whether you actually use the U type parameter at all.

This would be absurdly useful to support, especially if you could do so without needing a dummy type parameter, e.g.:
struct S<T> {
	func f<where T: Hashable>() -> Set<T>
}

I’m sure it’s complicated how this would interact with protocol requirements (e.g. if f is required by a conformed-to protocol, can it add extra constraints?), but no less useful for it. As it is you have to use free functions for everything like this.