_NSCFString must be marked @usableFromInline because _NSCFConstantString is.

tkremenek · tkremenek · commit 3107383fbf98 · 2018-11-15T10:14:45.000-08:00
diff --git a/Foundation/NSCFString.swift b/Foundation/NSCFString.swift
@@ -10,6 +10,7 @@
 
 import CoreFoundation
 
+@usableFromInline
 internal class _NSCFString : NSMutableString {
     required init(characters: UnsafePointer<unichar>, length: Int) {
         fatalError()
