Make EditableList<T> covariant

This makes `EditableList<IExpression>` a valid subtype of
`EditableList<EditableNode>`, which is safe as all HHAST node types are
immutable.

src/nodes/EditableList.hack

@@ -11,7 +11,7 @@ namespace Facebook\HHAST;
 
 use namespace HH\Lib\{C, Dict, Vec};
 
-final class EditableList<Titem as ?EditableNode> extends EditableNode {
+final class EditableList<+Titem as ?EditableNode> extends EditableNode {
   /**
    * Use `EditableList::createMaybeEmptyList()` or
    * `EditableList::createNonEmptyListOrMissing()` instead to be explicit

tests/NodeTypesTest.hack

@@ -0,0 +1,29 @@
+/*
+ *  Copyright (c) 2017-present, Facebook, Inc.
+ *  All rights reserved.
+ *
+ *  This source code is licensed under the MIT license found in the
+ *  LICENSE file in the root directory of this source tree.
+ *
+ */
+
+
+namespace Facebook\HHAST;
+
+final class NodeTypesTest extends TestCase {
+  public function testListVariance(): void {
+    $_typecheck = ((?EditableList<IParameter> $_) ==> null)(
+      (null ?as FunctionDeclarationHeader)?->getParameterListx(),
+    );
+    $_typecheck = ((?EditableList<EditableNode> $_) ==> null)(
+      (null ?as FunctionDeclarationHeader)?->getParameterListx(),
+    );
+  }
+
+  public function testBinaryExpressionOperandsAreExpressions(): void {
+    $typecheck = (?IExpression $_) ==> null;
+    $be = null ?as BinaryExpression;
+    $typecheck($be?->getLeftOperand());
+    $typecheck($be?->getRightOperand());
+  }
+}