[Sema] Properly diagnose Optional lowercase rename of None/Some

Because Optional static var .None and static func .Some unavailable
stubs that were put in the standard library aren't imported from
C/Objective-C, the path that diagnoses renaming enum cases to lowercase
isn't triggered.

rdar://problem/26481304

Author: bitjammer

lib/Sema/TypeCheckPattern.cpp

@@ -17,6 +17,7 @@
 
 #include "TypeChecker.h"
 #include "GenericTypeResolver.h"
+#include "swift/Basic/StringExtras.h"
 #include "swift/AST/ExprHandle.h"
 #include "swift/AST/ASTWalker.h"
 #include "swift/AST/ASTVisitor.h"
@@ -1401,9 +1402,24 @@ bool TypeChecker::coercePatternToType(Pattern *&P, DeclContext *dc, Type type,
                                       EEP->getLoc());
       }
       if (!elt) {
-        if (!type->is<ErrorType>())
+        if (!type->is<ErrorType>()) {
+          if (type->getAnyNominal() == Context.getOptionalDecl()) {
+            if (EEP->getName().str().compare("None") == 0 ||
+                EEP->getName().str().compare("Some") == 0) {
+              SmallString<4> Rename;
+              camel_case::toLowercaseWord(EEP->getName().str(), Rename);
+              diagnose(EEP->getLoc(), diag::availability_decl_unavailable_rename,
+                          EEP->getName(), /*replaced*/false,
+                          /*special kind*/0, Rename.str())
+                .fixItReplace(EEP->getLoc(), Rename.str());
+
+            }
+            return true;
+          }
+
           diagnose(EEP->getLoc(), diag::enum_element_pattern_member_not_found,
                    EEP->getName().str(), type);
+        }
         return true;
       }
       enumTy = type;

test/1_stdlib/OptionalRenames.swift

@@ -0,0 +1,16 @@
+// RUN: %target-parse-verify-swift
+
+func getInt(x: Int) -> Int? {
+  if x == 0 {
+    return .None // expected-error {{'None' has been renamed to 'none'}} {{13-17=none}}
+  }
+  return .Some(1) // expected-error {{'Some' has been renamed to 'some'}} {{11-15=some}}
+}
+
+let x = Optional.Some(1) // expected-error {{'Some' has been renamed to 'some'}} {{18-22=some}}
+
+switch x {
+  case .None: break // expected-error {{'None' has been renamed to 'none'}} {{9-13=none}}
+  case .Some(let x): print(x) // expected-error {{'Some' has been renamed to 'some'}} {{9-13=some}}
+}
+