An @objc enum needs a valid raw type to make a special hash(into:)

This check is superfluous in correct code, but necessary in the 4.2
branch to avoid failing in incorrect code. (On master, the error is
not reported because the enum is no longer considered @objc at this
point.)

Author: jrose-apple

lib/Sema/DerivedConformanceEquatableHashable.cpp

@@ -1233,12 +1233,18 @@ ValueDecl *DerivedConformance::deriveHashable(ValueDecl *requirement) {
 
       if (auto ED = dyn_cast<EnumDecl>(Nominal)) {
         void (*bodySynthesizer)(AbstractFunctionDecl *);
-        if (ED->isObjC())
+        if (ED->hasClangNode() ||
+            (ED->isObjC() && ED->hasRawType() &&
+             !ED->getInherited().front().isError())) {
+          // The checks for the raw type help avoid superfluous diagnostics in
+          // invalid code.
           bodySynthesizer = deriveBodyHashable_enum_rawValue_hashInto;
-        else if (ED->hasOnlyCasesWithoutAssociatedValues())
+        } else if (ED->hasOnlyCasesWithoutAssociatedValues()) {
           bodySynthesizer = deriveBodyHashable_enum_noAssociatedValues_hashInto;
-        else
-          bodySynthesizer=deriveBodyHashable_enum_hasAssociatedValues_hashInto;
+        } else {
+          bodySynthesizer =
+              deriveBodyHashable_enum_hasAssociatedValues_hashInto;
+        }
         return deriveHashable_hashInto(*this, bodySynthesizer);
       } else if (isa<StructDecl>(Nominal))
         return deriveHashable_hashInto(*this,