Type signature disambiguations do not take into account same-type generic constraints when substituting `Self`

[tayloraswift]

this is a hole that is originally caused by the Swift compiler bug swiftlang/swift#78343 , but is not sufficiently patched by Unidoc’s downstream heuristics.

specifically, given two extensions like these

struct S<T> {}

extension S<Int> 
{
    static var x:Self { get }
}
extension S<String> 
{
    static var x:Self { get }
}

the Swift compiler will fail to resolve the Self tokens, and Unidoc’s heuristic steps in, replacing Self with S<T>. however, this substitution has no disambiguative value, as it is identical between both extensions.

the client workaround is to avoid Self and spell these extensions using the actual resolved types:

extension S<Int> 
{
-   static var x:Self { get }
+   static var x:S<Int> { get }
}
extension S<String> 
{
-   static var x:Self { get }
+   static var x:S<String> { get }
}

this would allow documentation to disambiguate between the two overloads with

-  ``S.x -> S<Int>``
-  ``S.x -> S<String>``

in theory, Unidoc could inspect the generic constraints and substitute the type arguments itself. but at this point, we are layering heuristics upon heuristics in order to compensate for what is really a plain old Swift compiler bug.