こんな感じ。

/Volumes/Macintosh HD/Users/norio/github/IBPCollectionViewCompositionalLayout/Example/Example/Samples from Apple/Basics View Controllers/SectionDecorationViewController.swift:30:64: error: ambiguous use of 'fractionalWidth' let itemSize = NSCollectionLayoutSize(widthDimension: .fractionalWidth(1.0), ^ UIKit.NSCollectionLayoutDimension:4:21: note: found this candidate open class func fractionalWidth(_ fractionalWidth: CGFloat) -> Self ^ IBPCollectionViewCompositionalLayout.NSCollectionLayoutDimension:3:21: note: found this candidate open class func fractionalWidth(_ fractionalWidth: CGFloat) -> Self ^

NS_SWIFT_NAMEが漏れてたのでちょっと修正。 (edited)