CSGen: Infer generic arguments in explicit closure result types

Author: AnthonyLatsis

lib/Sema/CSGen.cpp

@@ -2073,12 +2073,14 @@ namespace {
 
         return resolveTypeReferenceInExpression(
             closure->getExplicitResultTypeRepr(),
-            TypeResolverContext::InExpression, nullptr);
+            TypeResolverContext::InExpression,
+            // Introduce type variables for unbound generics.
+            OpenUnboundGenericType(
+                CS, CS.getConstraintLocator(closure,
+                                            ConstraintLocator::ClosureResult)));
       };
 
       Type resultTy;
-      auto *resultLoc =
-            CS.getConstraintLocator(closure, ConstraintLocator::ClosureResult);
       if (auto explicityTy = getExplicitResultType()) {
         resultTy = explicityTy;
       } else {
@@ -2099,9 +2101,11 @@ namespace {
           // If this is a multi-statement closure, let's mark result
           // as potential hole right away.
           resultTy = CS.createTypeVariable(
-              resultLoc,
+              CS.getConstraintLocator(closure,
+                                      ConstraintLocator::ClosureResult),
               shouldTypeCheckInEnclosingExpression(closure)
-                ? 0 : TVO_CanBindToHole);
+                  ? 0
+                  : TVO_CanBindToHole);
         }
       }
 

test/Constraints/closures.swift

@@ -1030,3 +1030,15 @@ func sr12815() {
      .doesntExist2() { $0 }
   }
 }
+
+// Make sure we can infer generic arguments in an explicit result type.
+let explicitUnboundResult1 = { () -> Array in [0] }
+let explicitUnboundResult2: (Array<Bool>) -> Array<Int> = {
+  (arr: Array) -> Array in [0]
+}
+// FIXME: Should we prioritize the contextual result type and infer Array<Int>
+// rather than using a type variable in these cases?
+// expected-error@+1 {{unable to infer closure type in the current context}}
+let explicitUnboundResult3: (Array<Bool>) -> Array<Int> = {
+  (arr: Array) -> Array in [true]
+}