"Use of undeclared type" in Swift, even though type is internal, and exists in same module

I have a type in my module:

import Cocoa

class ColoredDotView : NSView {
   ...
}

It is used in a number of different classes with no issue:

class EditSubjectPopoverController : NSObject {

    @IBOutlet internal var subjectColorDotView : ColoredDotView!
    ...
}

But for some reason, when I use it in one specific class, I have compilation errors on the type:

class EditTaskPopoverController : NSObject {

    @IBOutlet internal var lowPriorityDotView : ColoredDotView! // Error here
    @IBOutlet internal var medPriorityDotView : ColoredDotView! // And here...
    @IBOutlet internal var highPriorityDotView : ColoredDotView! // And here...
    ...
}

The compilation error is:

EditTaskPopoverController.swift:15:49: Use of undeclared type 'ColoredDotView'

Which I don't understand. It's the first compilation error in the file, and the rest of the errors are all symptomatic of the first. Further, there are no other files with compilation errors. I don't understand why the type is undeclared, as the file is in the same module:

I have tried cleaning the project, cleaning the build folder, and restarting Xcode, to no avail. What potential missteps can cause an undeclared type compiler error in Swift?

For me, I encountered this error when my test target did not have some swift files that my app build target had in compile sources. It was very confusing because the 'undeclared type' was being used in so many other places with no problem, and the error seemed vague. So solution there was of course to add the file containing the 'undeclared type' to the test target.

This has already been answered by @Craig Otis, but the issue is caused when the classes in question do not belong to the same targets, usually the test target is missing. Just make sure the following check boxes are ticked.

Edit

To see the target membership. Select your file then open the file inspector (⌥ + ⌘ + 1) [option] + [command] + 1

Phew, finally diagnosed this. Somehow, the offending Swift file EditTaskPopoverController.swift was in two different build phases.

It was in Compile Sources properly, with all the other Swift files, but it was also, for some very strange reason, in the Copy Bundle Resources phase as well, along with all my XIB and image resources.

I have no idea how it got there, but removing it from the extra build phase resolved the issue.

In XCode menu Product->Clean and then Product->Build worked for me. I encountered this issue when added new ViewController to my project in new Group/Folder.