Conform Never to Error, Equatable, Comparable, and Hashable (#16857)

Author: mdiep

stdlib/public/core/Policy.swift

@@ -28,6 +28,19 @@
 @_frozen
 public enum Never {}
 
+extension Never: Error {}
+
+extension Never: Equatable {}
+
+extension Never: Comparable {
+  public static func < (lhs: Never, rhs: Never) -> Bool {
+    switch (lhs, rhs) {
+    }
+  }
+}
+
+extension Never: Hashable {}
+
 //===----------------------------------------------------------------------===//
 // Standardized aliases
 //===----------------------------------------------------------------------===//

test/stdlib/Never.swift

@@ -0,0 +1,13 @@
+// RUN: %target-typecheck-verify-swift
+
+struct ConformsToError<T: Error> {}
+_ = ConformsToError<Never>()
+
+struct ConformsToEquatable<T: Equatable> {}
+_ = ConformsToEquatable<Never>()
+
+struct ConformsToComparable<T: Comparable> {}
+_ = ConformsToComparable<Never>()
+
+struct ConformsToHashable<T: Hashable> {}
+_ = ConformsToHashable<Never>()