Reconcile fix with CFA work

Author: weswigham

src/compiler/checker.ts

@@ -7555,7 +7555,7 @@ namespace ts {
         }
 
         function getNarrowedTypeOfReference(type: Type, reference: Node) {
-            if (!(type.flags & TypeFlags.Narrowable) || !isNarrowableReference(reference)) {
+            if (type.flags & TypeFlags.Defaultable || !isNarrowableReference(reference)) {
                 return type;
             }
             const leftmostNode = getLeftmostIdentifierOrThis(reference);
@@ -7975,7 +7975,7 @@ namespace ts {
             const defaultsToDeclaredType = !strictNullChecks || type.flags & TypeFlags.Any || !declaration ||
                 declaration.kind === SyntaxKind.Parameter || isInAmbientContext(declaration) ||
                 getContainingFunctionOrModule(declaration) !== getContainingFunctionOrModule(node);
-            if (defaultsToDeclaredType && !(type.flags & TypeFlags.Narrowable)) {
+            if (defaultsToDeclaredType && type.flags & TypeFlags.Defaultable) {
                 return type;
             }
             const flowType = getFlowTypeOfReference(node, type, defaultsToDeclaredType ? type : undefinedType);

src/compiler/types.ts

@@ -2175,7 +2175,13 @@ namespace ts {
         ObjectType = Class | Interface | Reference | Tuple | Anonymous,
         UnionOrIntersection = Union | Intersection,
         StructuredType = ObjectType | Union | Intersection,
-        Narrowable = Any | ObjectType | Union | TypeParameter,
+
+        // 'Defaultable' types are types where narrowing reverts to the original type, rather than actually narrow.
+        // This is never really correct - you can _always_ narrow to an intersection with that type, _but_ we keep
+        // Void as the only defaultable type, since it's a non-value type construct (representing a lack of a value)
+        // and, generally speaking, narrowing `void` should fail in some way, as it is nonsensical. (`void` narrowing
+        // to `void & T`, in a structural sense, is just narrowing to T, which we wouldn't allow under normal rules)
+        Defaultable = Void,
         /* @internal */
         RequiresWidening = ContainsUndefinedOrNull | ContainsObjectLiteral,
         /* @internal */