Merge pull request #15110 from harlanhaskins/change-is-inevitable

Add #warning and #error description to Changelog

Author: DougGregor

CHANGELOG.md

@@ -23,6 +23,19 @@ CHANGELOG
 Swift 4.2
 ---------
 
+* [SE-0196][]
+  
+  Custom compile-time warnings or error messages can be emitted using the
+  `#warning(_:)` and `#error(_:)` directives.
+
+  ```swift
+  #warning("this is incomplete")
+
+  #if MY_BUILD_CONFIG && MY_OTHER_BUILD_CONFIG
+    #error("MY_BUILD_CONFIG and MY_OTHER_BUILD_CONFIG cannot both be set")
+  #endif
+  ```
+
 * Public classes may now have internal `required` initializers. The rule for
   `required` initializers is that they must be available everywhere the class
   can be subclassed, but previously we said that `required` initializers on