duplicates:

• SR-12817 Nested generic typealias extensions apply to the parent type.

Issue Description:

First, some code that does what I would expect:

struct Generic<T> {}
typealias Specific = Generic<Int>
 
extension Generic {
    func identify() { print("Any T") }
}
extension Specific {
    func identify() { print("T is Int") }
}
Generic<String>().identify()  // Any T
Generic<Int>().identify()     // T is Int
Specific().identify()         // T is Int

From this, it seems as if "extension Specific" is correctly interpreted as "extension Generic where T == Int".

However, the mechanism does not seem to generalize properly to typealiases that have only partial constraints:

struct Generic<A, B> {}
typealias Specific<T> = Generic<T, Int>
extension Generic {
    func identify() { print("Any B") }
}
 
extension Specific {
    func identify() { print("B is Int") }  // ERROR: Invalid redeclaration of 'identify()'
}

This error seems to suggest that "extension Specific" is interpreted identically to "extension Generic". The fact that Specific is Generic<_, Int> seems to have gotten lost.

Is this the intended behavior?