Open Radar

 
Summary:
When adding an extension to a type in Swift, one can repeat a generic type, but this can result in really confusing error messages.

Steps to Reproduce:
Try to compile the following code:

struct G<T> {}

extension G {
	static func g<T>() -> G<T> {
		return G()
	}
}

Expected Results:
Error message: “Function cannot redefine generic type ’T’”.

Actual Results:
Unhelpful and confusing error message: “'T' is not identical to ’T’”

Regression:
N/A

Notes: 
The code is obviously incorrect. One shouldn’t define <T> in g() again. But it would be nice to have a more accurate error message.