diff --git a/src/moduleutils/PhutilLibraryMapBuilder.php b/src/moduleutils/PhutilLibraryMapBuilder.php
--- a/src/moduleutils/PhutilLibraryMapBuilder.php
+++ b/src/moduleutils/PhutilLibraryMapBuilder.php
@@ -332,11 +332,16 @@
       'xmap'      => array(),
     );
 
+    $type_translation = array(
+      'interface' => 'class',
+      'trait' => 'class',
+    );
+
     // Detect duplicate symbols within the library.
     foreach ($symbol_map as $file => $info) {
       foreach ($info['have'] as $type => $symbols) {
         foreach ($symbols as $symbol => $declaration) {
-          $lib_type = ($type == 'interface') ? 'class' : $type;
+          $lib_type = idx($type_translation, $type, $type);
           if (!empty($library_map[$lib_type][$symbol])) {
             $prior = $library_map[$lib_type][$symbol];
             throw new Exception(
diff --git a/src/symbols/PhutilSymbolLoader.php b/src/symbols/PhutilSymbolLoader.php
--- a/src/symbols/PhutilSymbolLoader.php
+++ b/src/symbols/PhutilSymbolLoader.php
@@ -398,6 +398,12 @@
   }
 
 
+  private static function classLikeExists($name) {
+    return class_exists($name, false) ||
+           interface_exists($name, false) ||
+           trait_exists($name, false);
+  }
+
   /**
    * @task internal
    */
@@ -411,7 +417,7 @@
         return;
       }
     } else {
-      if (class_exists($name, false) || interface_exists($name, false)) {
+      if (self::classLikeExists($name)) {
         return;
       }
     }
@@ -431,7 +437,7 @@
         $load_failed = pht('function');
       }
     } else {
-      if (!class_exists($name, false) && !interface_exists($name, false)) {
+      if (!self::classLikeExists($name)) {
         $load_failed = pht('class or interface');
       }
     }
diff --git a/support/lib/extract-symbols.php b/support/lib/extract-symbols.php
--- a/support/lib/extract-symbols.php
+++ b/support/lib/extract-symbols.php
@@ -112,18 +112,6 @@
     $namespace, $path, pht('namespace `%s` statements', 'use'));
 }
 
-$possible_traits = $root->selectDescendantsOfType('n_CLASS_DECLARATION');
-foreach ($possible_traits as $possible_trait) {
-  $attributes = $possible_trait->getChildByIndex(0);
-  // Can't use getChildByIndex here because not all classes have attributes
-  foreach ($attributes->getChildren() as $attribute) {
-    if (strtolower($attribute->getConcreteString()) === 'trait') {
-      phutil_fail_on_unsupported_feature($possible_trait, $path, pht('traits'));
-    }
-  }
-}
-
-
 // -(  Marked Externals  )------------------------------------------------------
 
 
@@ -256,17 +244,29 @@
 
 // Find classes declared by this file.
 
-
 // This is "class X ... { ... }".
-$classes = $root->selectDescendantsOfType('n_CLASS_DECLARATION');
-foreach ($classes as $class) {
-  $class_name = $class->getChildByIndex(1);
-  $have[] = array(
-    'type'    => 'class',
+function build_have_element_for_class_declaration(XHPASTNode $class_node) {
+  $class_name = $class_node->getChildByIndex(1);
+
+  $type = 'class';
+  $attributes = $class_node->getChildByIndex(0);
+  foreach ($attributes->getChildren() as $attribute) {
+    if (strtolower($attribute->getConcreteString()) === 'trait') {
+      $type = 'trait';
+    }
+  }
+
+  return array(
+    'type'    => $type,
     'symbol'  => $class_name,
   );
 }
 
+$classes = $root->selectDescendantsOfType('n_CLASS_DECLARATION');
+foreach ($classes as $class) {
+  $have[] = build_have_element_for_class_declaration($class);
+}
+
 
 // Find classes used by this file. We identify these:
 //