Comment by Will Lisac (JIRA)

Using a protocol in the where clause is a possible workaround for this code completion issue.

class A<T> { }

extension A where T == String {
    // This method will always be visible in code completion
    func fooString() { }
}

extension A where T: StringProtocol {
    // This method will only be visible when T conforms to StringProtocol
    func fooStringProtocol() { }
}

let example = A<Bool>()
example.foo

The protocol-workaround only works when all code is in the same swift file, if you put the class and extensions and the calling code in separate files, it will list all extensions again.

Another very similar attempted workaround detailed example here will show the same behavior, ie working only if all code is in the same file.

I guess the following might be another manifestation of this issue:

let floatRange = Float(0) ..< Float(1)
floatRange.cou // ERROR: Value of type 'Range<Float>' has no member 'cou'; did you mean 'count'?
floatRange.count // ERROR: Type 'Float' does not conform to protocol 'SignedInteger'

I mean the supposedly helpful "did you mean `count`?" and the fact that `.count` is in the list of code completions, even though `Range<Float>` does not have a `.count` member.

This has been fixed in #23065

let floatRange = Float(0) ..< Float(1)
floatRange.cou // ERROR: Value of type 'Range<Float>' has no member 'cou'; did you mean 'count'?

This has been fixed in #23332

Thanks, Rintaro!

Is this supposed to be fixed in the default toolchain of Xcode 10.2.1?

I see Rintaro saying above that it has been fixed but the issue as described here (in SR-7046) and in the linked SR-9396 remains as of default toolchain of Xcode 10.2.1. That is, for example the following is still what happens in Xcode 10.2.1:

let floatRange = Float(0) ..< Float(1)
floatRange.cou // ERROR: Value of type 'Range<Float>' has no member 'cou'; did you mean 'count'?

even though Range<Float> has no property called count.

Not for Xcode 10.2.1 (Swift 5.0.1). Please wait for Swift 5.1.