Strongly prefer available declarations to unavailable ones in type checking.

Fixes rdar://problem/18847642, rdar://problem/16554496, and the
current 1_stdlib/Array.swift.

Swift SVN r25212

lib/Sema/CSRanking.cpp

@@ -37,6 +37,10 @@ void ConstraintSystem::increaseScore(ScoreKind kind) {
       log.indent(solverState->depth * 2);
     log << "(increasing score due to ";
     switch (kind) {
+    case SK_Unavailable:
+      log << "use of an unavailable declaration";
+      break;
+
     case SK_Fix:
       log << "attempting to fix the source";
       break;
@@ -671,17 +675,6 @@ SolutionCompareResult ConstraintSystem::compareSolutions(
         }
       }
 
-      // If one declaration is available and the other is not,
-      bool unavail1 = decl1->getAttrs().isUnavailable(cs.getASTContext());
-      bool unavail2 = decl2->getAttrs().isUnavailable(cs.getASTContext());
-      if (unavail1 != unavail2) {
-        if (unavail1)
-          ++score2;
-        else
-          ++score1;
-        continue;
-      }
-
       // If both declarations come from Clang, and one is a type and the other
       // is a function, prefer the function.
       if (decl1->hasClangNode() &&

lib/Sema/ConstraintSystem.cpp

@@ -1311,6 +1311,11 @@ void ConstraintSystem::resolveOverload(ConstraintLocator *locator,
       refType = ImplicitlyUnwrappedOptionalType::get(refType->getRValueType());
     } 
 
+    // If the declaration is unavailable, note that in the score.
+    if (choice.getDecl()->getAttrs().isUnavailable(getASTContext())) {
+      increaseScore(SK_Unavailable);
+    }
+
     break;
   }
 

lib/Sema/ConstraintSystem.h

@@ -705,6 +705,8 @@ struct SelectedOverload {
 enum ScoreKind {
   // These values are used as indices into a Score value.
 
+  /// A reference to an @unavailable declaration.
+  SK_Unavailable,
   /// A fix needs to be applied to the source.
   SK_Fix,
   /// An implicit force of an implicitly unwrapped optional value.

test/ClangModules/Inputs/custom-modules/ObjCParseExtras.h

@@ -104,3 +104,12 @@ __weak id globalWeakVar;
 @protocol FooDelegate <NSObject>
 @property (nonatomic, assign, readonly, getter=isStarted) BOOL started;
 @end
+
+// rdar://problem/18847642
+@interface NonNullDefaultInit
+-(nonnull instancetype)init;
+@end
+
+@interface NonNullDefaultInitSub : NonNullDefaultInit
++ (null_unspecified instancetype)sub;
+@end

test/ClangModules/objc_init.swift

@@ -144,3 +144,10 @@ class View: NSView {
     var p = MyViewController.init()
   }
 }
+
+// rdar://problem/19726164
+class NonNullDefaultInitSubSub : NonNullDefaultInitSub {
+  func foo() {
+    var x: NonNullDefaultInitSubSub? = NonNullDefaultInitSubSub()
+  }
+}

test/Constraints/overload.swift

@@ -106,3 +106,11 @@ struct HasX1aProperty {
     write(prop) // no error, not ambiguous
   }
 }
+
+// rdar://problem/16554496
+@availability(*, unavailable)
+func availTest(x: Int) {}
+func availTest(x: Any) { println("this one") }
+func doAvailTest(x: Int) {
+  availTest(x)
+}