Consider the following example.
```swift
protocol FontStyle {}
struct FontStyleOne: FontStyle {}
extension FontStyle where Self == FontStyleOne {
static var one: FontStyleOne { FontStyleOne() }
}
func foo<T: FontStyle>(x: T) {}
func case1() {
foo(x: .#^COMPLETE^#)
}
```
With SE-0299 accepted, we should be suggesting `.one`.
For that, we need to consider the extension applied when performing the unresolved dot code completion with a primary archetype that conforms to `FontStyle`.
However, in the following case, which performs an unresolved dot completion on the same base type, we don't want to suggest `.one` because that would require `T == FontStyleOne`, which we can’t assume.
```swift
func case2<T: FontStyle>(x: T) {
x.#^COMPLETE_2^#
}
```
Since the constraint system cannot tell us how it came up with the archetype, we need to apply a heuristic to differentiate between the two cases.
What seems to work fine in most cases, is to determine if `T` referes to a generic parameter that is visible from the decl context we are completing in (i.e. the decl context we are completing in is a child context of the context that `T` is declared in). If it is not, then `T` cannot be the type of a variable we are completing on. Thus, we are in the first case and we should consider all extensions of `FontStyle` because we can further specialize `T` by picking a more concrete type.
Otherwise `T` may be the type of a variable we are completing on and we should be conservative and only suggest those extensions whose requirements are fulfilled by `T`.
Since this is just a heuristic, there are some corner cases, where we aren’t suggesting constrainted extensions although we should. For example, in the following example the call to `testRecursive` doesn’t use `T` and we should thus suggest `one`. But by the rules described above we detect that `T` is accessible at the call and thus don’t apply extension whose requirements aren’t satisfied.
```swift
func testRecursive<T: FontStyle>(_ style: T) {
testRecursive(.#^COMPLETE_RECURSIVE_GENERIC^#)
}
```
Similar completion issues also occurred without SE-0299 in more complicated, generic scenarios.
Resolves rdar://74958497 and SR-12973
