Index: src/__phutil_library_map__.php
===================================================================
--- src/__phutil_library_map__.php
+++ src/__phutil_library_map__.php
@@ -139,6 +139,7 @@
     'PhutilDefaultSyntaxHighlighterEngineTestCase' => 'markup/syntax/engine/__tests__/PhutilDefaultSyntaxHighlighterEngineTestCase.php',
     'PhutilDeferredLog' => 'filesystem/PhutilDeferredLog.php',
     'PhutilDeferredLogTestCase' => 'filesystem/__tests__/PhutilDeferredLogTestCase.php',
+    'PhutilDirectedScalarGraph' => 'utils/PhutilDirectedScalarGraph.php',
     'PhutilDirectoryFixture' => 'filesystem/PhutilDirectoryFixture.php',
     'PhutilDivinerSyntaxHighlighter' => 'markup/syntax/highlighter/PhutilDivinerSyntaxHighlighter.php',
     'PhutilDocblockParser' => 'parser/PhutilDocblockParser.php',
@@ -519,6 +520,7 @@
     'PhutilDefaultSyntaxHighlighterEnginePygmentsFuture' => 'FutureProxy',
     'PhutilDefaultSyntaxHighlighterEngineTestCase' => 'PhutilTestCase',
     'PhutilDeferredLogTestCase' => 'PhutilTestCase',
+    'PhutilDirectedScalarGraph' => 'AbstractDirectedGraph',
     'PhutilDocblockParserTestCase' => 'PhutilTestCase',
     'PhutilEditDistanceMatrixTestCase' => 'PhutilTestCase',
     'PhutilEmailAddressTestCase' => 'PhutilTestCase',
Index: src/utils/PhutilDirectedScalarGraph.php
===================================================================
--- /dev/null
+++ src/utils/PhutilDirectedScalarGraph.php
@@ -0,0 +1,15 @@
+<?php
+
+/**
+ * Concrete subclass of @{class:AbstractDirectedGraph} which can not load any
+ * data from external sources.
+ */
+final class PhutilDirectedScalarGraph extends AbstractDirectedGraph {
+
+  protected function loadEdges(array $nodes) {
+    throw new Exception(
+      "PhutilDirectedScalarGraph can not load additional nodes at runtime. ".
+      "Tried to load: ".implode(', ', $nodes));
+  }
+
+}