[Name lookup] Don't allow unavailable-in-current-Swift declarations to shadow available ones.

The unavailable-in-current-Swift declarations introduced by the Clang
importer to help with migrating Swift 2.x code to Swift 3's naming
cause problems with unqualified name lookup when they shadow, e.g.,
types. The biggest problem in practice is with "URL", which is a
common Cocoa property name (in Swift 2) that becomes "url" in Swift 3,
but the old name conflicts with the Foundation value type "URL".

Fixes <rdar://problem/26236989>.

Author: DougGregor

lib/AST/NameLookup.cpp

@@ -391,6 +391,8 @@ UnqualifiedLookup::UnqualifiedLookup(DeclName Name, DeclContext *DC,
   if (IsKnownNonCascading)
     isCascadingUse = false;
 
+  SmallVector<UnqualifiedLookupResult, 4> UnavailableInnerResults;
+
   // Never perform local lookup for operators.
   if (Name.isOperator()) {
     if (!isCascadingUse.hasValue()) {
@@ -565,9 +567,25 @@ UnqualifiedLookup::UnqualifiedLookup(DeclName Name, DeclContext *DC,
         }
 
         if (FoundAny) {
-          if (DebugClient)
-            filterForDiscriminator(Results, DebugClient);
-          return;
+          // Predicate that determines whether a lookup result should
+          // be unavailable except as a last-ditch effort.
+          auto unavailableLookupResult =
+            [&](const UnqualifiedLookupResult &result) {
+            return result.getValueDecl()->getAttrs()
+                     .isUnavailableInCurrentSwift();
+          };
+
+          // If all of the results we found are unavailable, keep looking.
+          if (std::all_of(Results.begin(), Results.end(),
+                          unavailableLookupResult)) {
+            UnavailableInnerResults.append(Results.begin(), Results.end());
+            Results.clear();
+            FoundAny = false;
+          } else {
+            if (DebugClient)
+              filterForDiscriminator(Results, DebugClient);
+            return;
+          }
         }
 
         // Check the generic parameters if our context is a generic type or
@@ -641,6 +659,14 @@ UnqualifiedLookup::UnqualifiedLookup(DeclName Name, DeclContext *DC,
   if (!Results.empty())
     return;
 
+  // If we still haven't found anything, but we do have some
+  // declarations that are "unavailable in the current Swift", drop
+  // those in.
+  if (!UnavailableInnerResults.empty()) {
+    Results = std::move(UnavailableInnerResults);
+    return;
+  }
+
   if (!Name.isSimpleName())
     return;
 

test/ClangModules/swift2_warnings.swift

@@ -5,6 +5,7 @@
 
 import Foundation
 import ImportAsMember.A
+import AppKit
 
 func testOldTypeNames() {
   var ps: NSPostingStyle? // expected-error{{'NSPostingStyle' has been renamed to 'PostingStyle'}}{{11-25=PostingStyle}}
@@ -77,3 +78,16 @@ func testImportAsMember() {
   IAMStruct1SetRadius(iam1, 3.14159)
   // expected-error@-1{{'IAMStruct1SetRadius' has been replaced by property 'Struct1.radius'}}{{3-22=iam1.radius}} {{22-29= = }} {{36-37=}}
 }
+
+// rdar://problem/26236989
+class X : NSDocument {
+  func test(url: URL) {
+  }
+  func test2() {
+    let url = URL(string: "ABC")
+    self.url = url!
+  }
+  func getTheURL() -> URL {
+    return url
+  }
+}

test/Inputs/clang-importer-sdk/usr/include/AppKit.h

@@ -300,3 +300,7 @@ typedef NSPoint *NSPointPointer;
 - (void)addRect:(NSRect)rect;
 + (void)conjureRect:(NSRect)rect;
 @end
+
+@interface NSDocument (URL)
+@property (copy,nonnull) NSURL *URL;
+@end