Sema: Don't allow closure return type to contain noescape types

Author: slavapestov

lib/Sema/CSGen.cpp

@@ -2404,7 +2404,7 @@ namespace {
           CS.getConstraintLocator(expr, ConstraintLocator::ClosureResult);
 
         if (expr->hasEmptyBody()) {
-          resultTy = CS.createTypeVariable(locator, TVO_CanBindToNoEscape);
+          resultTy = CS.createTypeVariable(locator, 0);
 
           // Closures with empty bodies should be inferred to return
           // ().
@@ -2416,7 +2416,7 @@ namespace {
         } else {
           // If no return type was specified, create a fresh type
           // variable for it.
-          resultTy = CS.createTypeVariable(locator, TVO_CanBindToNoEscape);
+          resultTy = CS.createTypeVariable(locator, 0);
 
           if (closureHasNoResult(expr)) {
             // Allow it to default to () if there are no return statements.

test/Constraints/closures.swift

@@ -899,3 +899,10 @@ do {
     }
   }
 }
+
+// Don't allow result type of a closure to end up as a noescape type
+
+// The funny error is because we infer the type of badResult as () -> ()
+// via the 'T -> U => T -> ()' implicit conversion.
+let badResult = { (fn: () -> ()) in fn }
+// expected-error@-1 {{expression resolves to an unused function}}