[638][cling] Fix infinite recursion when printing self-referencing containers

interpreter/cling/include/cling/Interpreter/RuntimePrintValue.h

@@ -210,10 +210,20 @@ namespace cling {
       const void* M = TypeTest::isMap(obj);
 
       std::string str("{ ");
-      str += printValue(&(*iter), M);
+
+      // If the dereferenced iterator points to the container itself, we have
+      // infinite recursion. This occurs with scalar values in nlohmann::json
+      auto printWithRecursionGuard = [obj](const auto* ptr,
+                                           const void* M) -> std::string {
+        if (static_cast<const void*>(ptr) == static_cast<const void*>(obj))
+          return "<recursion detected>";
+        return printValue(ptr, M);
+      };
+
+      str += printWithRecursionGuard(&(*iter), M);
       while (++iter != iterEnd) {
         str += ", ";
-        str += printValue(&(*iter), M);
+        str += printWithRecursionGuard(&(*iter), M);
       }
       return str + " }";
     }

interpreter/cling/test/Prompt/ValuePrinter/Recursion.C

@@ -0,0 +1,30 @@
+//------------------------------------------------------------------------------
+// CLING - the C++ LLVM-based InterpreterG :)
+//
+// This file is dual-licensed: you can choose to license it under the University
+// of Illinois Open Source License or the GNU Lesser General Public License. See
+// LICENSE.TXT for details.
+
+//------------------------------------------------------------------------------
+
+// RUN: cat %s | %cling -Xclang -verify 2>&1 | FileCheck %s
+
+.rawInput 1
+// When begin() != end() but *begin() points to the container itself
+// (in nlohmann::json), printValue_impl infinitely recurses without the
+// self-reference check.
+
+class RecursionTest {
+public:
+    RecursionTest() = default;
+    auto begin() const { return this; } // iterate over self
+    auto end() const { return this + 1; } // just to make sure begin() != end()
+};
+.rawInput 0
+
+RecursionTest j;
+j
+// CHECK: (RecursionTest &) { <recursion detected> }
+
+// expected-no-diagnostics
+.q