Maintain a list of visited FQNs while following extends to avoid loops.

fixtures/self_referencing_class.php

@@ -1,4 +1,20 @@
 <?php
-class C extends C {}
-$c = new C;
-$c->undef_prop = 1;
+namespace RecursiveTest;
+
+class A extends A {}
+
+class B extends C {}
+class C extends B {}
+
+class D extends E {}
+class E extends F {}
+class F extends D {}
+
+$a = new A;
+$a->undef_prop = 1;
+
+$b = new B;
+$b->undef_prop = 1;
+
+$d = new D;
+$d->undef_prop = 1;

src/DefinitionResolver.php

@@ -437,6 +437,7 @@ class DefinitionResolver
 
         // Find the right class that implements the member
         $implementorFqns = [$classFqn];
+        $visitedFqns = [];
 
         while ($implementorFqn = array_shift($implementorFqns)) {
             // If the member FQN exists, return it
@@ -449,10 +450,13 @@ class DefinitionResolver
             if ($implementorDef === null) {
                 break;
             }
+            // Note the FQN as visited
+            $visitedFqns[] = $implementorFqn;
             // Repeat for parent class
             if ($implementorDef->extends) {
                 foreach ($implementorDef->extends as $extends) {
-                    if ($extends !== $implementorFqn) {
+                    // Don't add the parent FQN if it's already been visited
+                    if (!\in_array($extends, $visitedFqns)) {
                         $implementorFqns[] = $extends;
                     }
                 }