Extensions of generic type aliases are not fully implemented

[rintaro]

Previous ID	SR-4875
Radar	None
Original Reporter	@rintaro
Type	Bug

Environment

Xcode8.3.2

Additional Detail from JIRA

Votes	0
Component/s	Compiler
Labels	Bug, AcceptsInvalid
Assignee	@slavapestov
Priority	Medium

md5: f1ef05c08a6589e24bf54d7f026401f0

Issue Description:

struct Foo<K,V> {
    var k: K
    var v: V
}

typealias IntFoo<V> = Foo<Int, V>

extension IntFoo where V == Int {
    func f() {
        print(k)
    }
}

Foo(k: "test", v: 1).f()

This should be rejected as:
constrained extension must be declared on the unspecialized generic type 'Foo' with constraints specified by a 'where' clause

[belkadan]

@slavapestov, we could probably make this a StarterBug, right? It's just one extra check at the extension site to make sure the typealias is either non-generic or unspecialized.

[belkadan]

…that's just a warning in Swift 3 mode, I guess.