NFC: Test derived Comparable conformances for enums with unavailable cases.

Also, add a FIXME to track improving the diagnostics when derivation of a
Comparable conformance is blocked by unavailable enum cases.

Author: tshortli

Diff for: lib/Sema/DerivedConformanceComparable.cpp

@@ -342,5 +342,7 @@ void DerivedConformance::tryDiagnoseFailedComparableDerivation(
                          rawType, nominal->getDeclaredInterfaceType(),
                          comparableProto->getDeclaredInterfaceType());
     }
+    // FIXME: Diagnose potentially unavailable enum elements that are preventing
+    // Comparable synthesis.
   }
 }

Diff for: test/decl/protocol/special/comparable/comparable_unsupported.swift

@@ -24,6 +24,28 @@ enum NotComparableEnumOne: Int, Comparable {
   case value
 }
 
+// A potentially unavailable (or unconditionally unavailable) enum case prevents
+// automatic synthesis of Comparable requirements.
+// FIXME: This should be diagnosed explicitly.
+
+enum EnumWithUnavailableCase: Comparable {
+  // expected-error@-1 {{type 'EnumWithUnavailableCase' does not conform to protocol 'Comparable'}}
+  case available
+
+  @available(*, unavailable)
+  case unavailable
+}
+
+enum EnumWithUnavailableCaseAndAssociatedValue: Comparable {
+  // expected-error@-1 {{type 'EnumWithUnavailableCaseAndAssociatedValue' does not conform to protocol 'Comparable'}}
+  enum SomeComparable: Comparable {}
+
+  case none
+
+  @available(*, unavailable)
+  case some(SomeComparable)
+}
+
 // Automatic synthesis of Comparable requires associated values to be Comparable as well.
 
 enum NotComparableEnumTwo: Comparable {