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Issue Description:

I have a framework target set to deploy on iOS 8. The code is attached as a zip archive.

An iOS 9+ type has been extended in said framework and looks like so:

@available(iOS 9.0, *)
extension NSLayoutAnchor {

    open func constrainEqual(_ anchor: NSLayoutAnchor, constant: CGFloat = 0) {
        constraint(equalTo: anchor, constant: constant).isActive = true
    }

}

When attempting to import this framework into an iOS 8+ App target, I get a problem.

:0: error: could not build Objective-C module '' expected a type.

The error is referring to the AnchorType, which appears to be wrapped in /* */ in the framework's generated header file.

@interface NSLayoutAnchor (SWIFT_EXTENSION(<module name>))
- (void)constrainEqual:(NSLayoutAnchor</* AnchorType */> * _Nonnull)anchor constant:(CGFloat)constant;
@end