[QoI] Improve contextual mismatch diagnostics in calls with trailing closures

Resolves: rdar://problem/28012273

Author: xedin

lib/Sema/CSDiag.cpp

@@ -6098,10 +6098,17 @@ bool FailureDiagnosis::diagnoseTrailingClosureErrors(ApplyExpr *callExpr) {
   // let's try to re-typecheck it to see if we can get some
   // more information about what is going on.
   if (currentType->hasTypeVariable() || currentType->hasUnresolvedType()) {
-    CallResultListener listener(CS->getContextualType());
+    auto contextualType = CS->getContextualType();
+    CallResultListener listener(contextualType);
     CS->TC.getPossibleTypesOfExpressionWithoutApplying(
         fnExpr, CS->DC, possibleTypes, FreeTypeVariableBinding::UnresolvedType,
         &listener);
+
+    // Looks like there is there a contextual mismatch
+    // related to function type, let's try to diagnose it.
+    if (possibleTypes.empty() && contextualType &&
+        !contextualType->hasUnresolvedType())
+      return diagnoseContextualConversionError();
   } else {
     possibleTypes.push_back(currentType);
   }

lib/Sema/ConstraintSystem.h

@@ -752,7 +752,7 @@ enum class ConstraintSystemFlags {
   PreferForceUnwrapToOptional = 0x02,
 
   /// If set, this is going to prevent constraint system from erasing all
-  /// discovered solutions expect the best one.
+  /// discovered solutions except the best one.
   ReturnAllDiscoveredSolutions = 0x04,
 };
 

test/Constraints/trailing_closures_objc.swift

@@ -18,3 +18,12 @@ func rdar28004686(a: [IndexPath]) {
   _ = a.sorted { (lhs: NSIndexPath, rhs: NSIndexPath) -> Bool in true }
   // expected-error@-1 {{'NSIndexPath' is not convertible to 'IndexPath'}}
 }
+
+class Test: NSObject {
+  var categories : NSArray?
+  func rdar28012273() {
+    let categories = ["hello", "world"]
+    self.categories = categories.sorted { $0.localizedCaseInsensitiveCompare($1) == ComparisonResult.orderedDescending }
+    // expected-error@-1 {{cannot assign value of type '[String]' to type 'NSArray?'}} {{121-121= as NSArray}}
+  }
+}