These changes implement some oft-requested tweaks and fixes to our closure implementation:

- Closures that are comprised of only a single return statement are now considered to be "single expression" closures. (rdar://problem/17550847)
- Unannotated single expression closures with non-void return types can now be used in void contexts. (rdar://problem/17228969)
- Situations where a multi-statement closure's type could not be inferred because of the lack of a return-type annotation are now properly diagnosed. (rdar://problem/17212107)

I also encountered a number of crashers along the way, which should now be fixed.

Swift SVN r24817

lib/Sema/CSSimplify.cpp

@@ -1961,6 +1961,22 @@ ConstraintSystem::matchTypes(Type type1, Type type2, TypeMatchKind kind,
                           ConversionRestrictionKind::ForceUnchecked);
     }
   }
+  
+  // If the types disagree, but we're comparing a non-void, single-expression
+  // closure result type to a void function result type, allow the conversion.
+  {
+    if (concrete && kind >= TypeMatchKind::Subtype && type2->isVoid()) {
+      SmallVector<LocatorPathElt, 2> parts;
+      locator.getLocatorParts(parts);
+      
+      while (!parts.empty()) {
+        if (parts.back().getKind() == ConstraintLocator::ClosureResult) {
+          return SolutionKind::Solved;
+        }
+        parts.pop_back();
+      }
+    }
+  }
 
 commit_to_conversions:
   // When we hit this point, we're committed to the set of potential