cpyrt: null-guard the cling.printValue lookup in op_str

Commit 5a33027 dropped the faked cling::runtime::gCling and with it the
last cling namespace in the interpreter. The pretty-print fallback in
op_str then dereferenced the failed cppjit.gbl.cling lookup. str() of
any instance without an ostream inserter crashed with SIGSEGV. Guard
both lookups, clear the AttributeError, and fall back to the generic
repr.

The regression test runs the repro in a subprocess, so a return of the
crash cannot kill the test runner. A clean exit passes, whichever repr
form the backend prints. Output that names the upstream toString stub
xfails. Anything else fails and reports the child's output.

Co-developed-with-the-help-of: Claude Code (Fable 5, human in the loop)

Author: Emery Conrad

src/cpyrt/CPPInstance.cxx

@@ -875,17 +875,22 @@ static PyObject* op_str(CPPInstance* self) {
     if (!printValue) {
       PyObject* gbl =
           PyDict_GetItemString(PySys_GetObject((char*)"modules"), "cppjit.gbl");
-      PyObject* cl = PyObject_GetAttrString(gbl, (char*)"cling");
-      printValue = PyObject_GetAttrString(cl, (char*)"printValue");
-      Py_DECREF(cl);
+      // no cling namespace exists unless user code declares one
+      PyObject* cl =
+          gbl ? PyObject_GetAttrString(gbl, (char*)"cling") : nullptr;
+      printValue =
+          cl ? PyObject_GetAttrString(cl, (char*)"printValue") : nullptr;
+      Py_XDECREF(cl);
       // gbl is borrowed
       if (printValue) {
         Py_DECREF(printValue); // make borrowed
         if (!PyCallable_Check(printValue))
           printValue = nullptr; // unusable ...
       }
-      if (!printValue) // unlikely
+      if (!printValue) {
+        PyErr_Clear();
         ScopeFlagSet(self, CPPScope::kNoPrettyPrint);
+      }
     }
 
     if (printValue) {

test/test_regression.py

@@ -1,7 +1,7 @@
 import os
 import sys
 
-from pytest import mark, raises, skip
+from pytest import mark, raises, skip, xfail
 from support import (
     IS_CLANG_REPL,
     IS_CLING,
@@ -1637,3 +1637,50 @@ def test51_nontype_enum_template_arg(self):
 
         # ...nor leave the interpreter unable to compile a later call wrapper
         assert ns.probe(41) == 42
+
+    def test52_str_fallback_without_ostream_insertion(self):
+        """str() of an instance with no operator<< used to crash.
+
+        With no ``cling`` namespace in the interpreter, the pretty-print
+        fallback dereferenced the failed ``cppjit.gbl.cling`` lookup and the
+        process died. A regression is therefore fatal, not an assertion
+        failure, so run the repro in a subprocess: the runner survives and the
+        output identifies which failure happened.
+        """
+
+        import subprocess
+        import sys
+
+        repro = """\
+import cppjit
+
+cppjit.cppdef("namespace StrFallback { struct Bare { int x; }; }")
+print(repr(str(cppjit.gbl.StrFallback.Bare())))
+"""
+
+        popen = subprocess.Popen(
+            [sys.executable, "-c", repro],
+            stdout=subprocess.PIPE,
+            stderr=subprocess.STDOUT,
+        )
+        stdout, _ = popen.communicate()
+        output = stdout.decode("utf-8", "replace")
+
+        # the guard holds: cling prints the @0xADDR form through printValue and
+        # ClangRepl falls back to the generic repr, and neither crashes
+        if popen.returncode == 0:
+            return
+
+        # Interpreter::toString is an assert(0) stub upstream. str() tries the
+        # ostream path first, which reaches it whenever assertions are on.
+        if "toString is not implemented" in output:
+            xfail(
+                "toString stub aborts, see compiler-research/CppInterOp#1100: "
+                "%s" % (output[:300],)
+            )
+
+        # a crash banner and its top frames come first, so keep the head
+        raise AssertionError(
+            "str() without an ostream inserter did not fall back cleanly: "
+            "returncode=%s output=%r" % (popen.returncode, output[:2000])
+        )