python
diff --git a/Lib/test/test_cppext/__init__.py b/Lib/test/test_cppext/__init__.py
@@ -24,34 +24,12 @@
 @support.requires_venv_with_pip()
 @support.requires_subprocess()
 @support.requires_resource('cpu')
-class TestCPPExt(unittest.TestCase):
+class BaseTests:
+    TEST_INTERNAL_C_API = False
+
     def test_build(self):
         self.check_build('_testcppext')
 
-    def test_build_cpp03(self):
-        # In public docs, we say C API is compatible with C++11. However,
-        # in practice we do maintain C++03 compatibility in public headers.
-        # Please ask the C API WG before adding a new C++11-only feature.
-        self.check_build('_testcpp03ext', std='c++03')
-
-    @support.requires_gil_enabled('incompatible with Free Threading')
-    def test_build_limited_cpp03(self):
-        self.check_build('_test_limited_cpp03ext', std='c++03', limited=True)
-
-    @unittest.skipIf(support.MS_WINDOWS, "MSVC doesn't support /std:c++11")
-    def test_build_cpp11(self):
-        self.check_build('_testcpp11ext', std='c++11')
-
-    # Only test C++14 on MSVC.
-    # On s390x RHEL7, GCC 4.8.5 doesn't support C++14.
-    @unittest.skipIf(not support.MS_WINDOWS, "need Windows")
-    def test_build_cpp14(self):
-        self.check_build('_testcpp14ext', std='c++14')
-
-    @support.requires_gil_enabled('incompatible with Free Threading')
-    def test_build_limited(self):
-        self.check_build('_testcppext_limited', limited=True)
-
     def check_build(self, extension_name, std=None, limited=False):
         venv_dir = 'env'
         with support.setup_venv_with_pip_setuptools(venv_dir) as python_exe:
@@ -71,6 +49,7 @@ def run_cmd(operation, cmd):
             if limited:
                 env['CPYTHON_TEST_LIMITED'] = '1'
             env['CPYTHON_TEST_EXT_NAME'] = extension_name
+            env['TEST_INTERNAL_C_API'] = str(int(self.TEST_INTERNAL_C_API))
             if support.verbose:
                 print('Run:', ' '.join(map(shlex.quote, cmd)))
                 subprocess.run(cmd, check=True, env=env)
@@ -111,5 +90,35 @@ def run_cmd(operation, cmd):
         run_cmd('Import', cmd)
 
 
+class TestPublicCAPI(BaseTests, unittest.TestCase):
+    @support.requires_gil_enabled('incompatible with Free Threading')
+    def test_build_limited_cpp03(self):
+        self.check_build('_test_limited_cpp03ext', std='c++03', limited=True)
+
+    @support.requires_gil_enabled('incompatible with Free Threading')
+    def test_build_limited(self):
+        self.check_build('_testcppext_limited', limited=True)
+
+    def test_build_cpp03(self):
+        # In public docs, we say C API is compatible with C++11. However,
+        # in practice we do maintain C++03 compatibility in public headers.
+        # Please ask the C API WG before adding a new C++11-only feature.
+        self.check_build('_testcpp03ext', std='c++03')
+
+    @unittest.skipIf(support.MS_WINDOWS, "MSVC doesn't support /std:c++11")
+    def test_build_cpp11(self):
+        self.check_build('_testcpp11ext', std='c++11')
+
+    # Only test C++14 on MSVC.
+    # On s390x RHEL7, GCC 4.8.5 doesn't support C++14.
+    @unittest.skipIf(not support.MS_WINDOWS, "need Windows")
+    def test_build_cpp14(self):
+        self.check_build('_testcpp14ext', std='c++14')
+
+
+class TestInteralCAPI(BaseTests, unittest.TestCase):
+    TEST_INTERNAL_C_API = True
+
+
 if __name__ == "__main__":
     unittest.main()
diff --git a/Lib/test/test_cppext/extension.cpp b/Lib/test/test_cppext/extension.cpp
@@ -6,8 +6,24 @@
 // Always enable assertions
 #undef NDEBUG
 
+#ifdef TEST_INTERNAL_C_API
+#  define Py_BUILD_CORE_MODULE 1
+#endif
+
 #include "Python.h"
 
+#ifdef TEST_INTERNAL_C_API
+   // gh-135906: Check for compiler warnings in the internal C API
+#  include "internal/pycore_frame.h"
+   // mimalloc emits many compiler warnings when Python is built in debug
+   // mode (when MI_DEBUG is not zero).
+   // mimalloc emits compiler warnings when Python is built on Windows.
+#  if !defined(Py_DEBUG) && !defined(MS_WINDOWS)
+#    include "internal/pycore_backoff.h"
+#    include "internal/pycore_cell.h"
+#  endif
+#endif
+
 #ifndef MODULE_NAME
 #  error "MODULE_NAME macro must be defined"
 #endif

diff --git a/Lib/test/test_cppext/setup.py b/Lib/test/test_cppext/setup.py
@@ -47,6 +47,7 @@ def main():
     std = os.environ.get("CPYTHON_TEST_CPP_STD", "")
     module_name = os.environ["CPYTHON_TEST_EXT_NAME"]
     limited = bool(os.environ.get("CPYTHON_TEST_LIMITED", ""))
+    internal = bool(int(os.environ.get("TEST_INTERNAL_C_API", "0")))
 
     cppflags = list(CPPFLAGS)
     cppflags.append(f'-DMODULE_NAME={module_name}')
@@ -82,6 +83,9 @@ def main():
         version = sys.hexversion
         cppflags.append(f'-DPy_LIMITED_API={version:#x}')
 
+    if internal:
+        cppflags.append('-DTEST_INTERNAL_C_API=1')
+
     # On Windows, add PCbuild\amd64\ to include and library directories
     include_dirs = []
     library_dirs = []