gh-86298: Ensure that __loader__ and __spec__.loader agree in warnings.warn_explicit()

Doc/reference/import.rst

@@ -552,6 +552,11 @@ the module.
    for introspection, but can be used for additional loader-specific
    functionality, for example getting data associated with a loader.
 
+   .. versionchanged:: 3.12
+      The value of ``__loader__`` is expected to be the same as
+      ``__spec__.loader``.  If they are not, an :exc:`ImportWarning` will be
+      raised.
+
 .. attribute:: __package__
 
    The module's ``__package__`` attribute must be set.  Its value must
@@ -563,8 +568,7 @@ the module.
    details.
 
    This attribute is used instead of ``__name__`` to calculate explicit
-   relative imports for main modules, as defined in :pep:`366`. It is
-   expected to have the same value as ``__spec__.parent``.
+   relative imports for main modules, as defined in :pep:`366`.
 
    .. versionchanged:: 3.6
       The value of ``__package__`` is expected to be the same as

Lib/test/test_warnings/__init__.py

@@ -5,6 +5,7 @@
 import re
 import sys
 import textwrap
+from types import ModuleType, SimpleNamespace
 import unittest
 from test import support
 from test.support import import_helper
@@ -885,6 +886,29 @@ def test_issue31566(self):
                  support.swap_item(globals(), '__file__', None):
                 self.assertRaises(UserWarning, self.module.warn, 'bar')
 
+    @support.cpython_only
+    def test_gh86298(self):
+        # warn_explicit() itself issues an ImportWarning when a module and
+        # module_globals are both given, and the named module has a __loader__
+        # != its __spec__.loader.
+        bar = ModuleType('bar')
+        # First, make the __loader__ != the __spec__.loader.  This should
+        # trigger the ImportWarning inside warn_explicit().
+        bar.__loader__ = object()
+        bar.__spec__ = SimpleNamespace(loader=object())
+        with original_warnings.catch_warnings():
+            self.assertWarns(
+                ImportWarning, self.module.warn_explicit,
+                'warning!', RuntimeWarning,
+                'bar.py', 2, module='bar knee', module_globals=bar.__dict__)
+        # Now make the __loader__ == __spec__.loader.  This will not raise the exception.
+        bar.__spec__.loader = bar.__loader__
+        with original_warnings.catch_warnings():
+            self.module.filterwarnings('error', category=ImportWarning)
+            self.module.warn_explicit(
+                'warning!', RuntimeWarning,
+                'bar.py', 2, module='bar knee', module_globals=bar.__dict__)
+
 
 class WarningsDisplayTests(BaseTest):
 

Misc/NEWS.d/next/Core and Builtins/2022-10-04-14-04-40.gh-issue-86298.QVM7G1.rst

@@ -0,0 +1,3 @@
+In cases where ``warnings.warn_explicit()`` consults the module's loader, an
+``ImportWarning`` is issued when ``m.__loader__`` differs from
+``m.__spec__.loader``.

Python/_warnings.c

@@ -977,25 +977,59 @@ warnings_warn_impl(PyObject *module, PyObject *message, PyObject *category,
 static PyObject *
 get_source_line(PyInterpreterState *interp, PyObject *module_globals, int lineno)
 {
-    PyObject *loader;
+    PyObject *loader, *spec_loader;
+    PyObject *spec;
     PyObject *module_name;
     PyObject *get_source;
     PyObject *source;
     PyObject *source_list;
     PyObject *source_line;
 
-    /* Check/get the requisite pieces needed for the loader. */
+    module_name = _PyDict_GetItemWithError(module_globals, &_Py_ID(__name__));
+    if (!module_name) {
+        return NULL;
+    }
+    Py_INCREF(module_name);
+
+    /* Check/get the requisite pieces needed for the loader.  Get both the
+       __spec__.loader and __loader__ attributes and warn if they are not the
+       same.
+    */
+    spec = _PyDict_GetItemWithError(module_globals, &_Py_ID(__spec__));
+    if (spec == NULL) {
+        Py_DECREF(module_name);
+        return NULL;
+    }
+    Py_INCREF(spec);
+
+    spec_loader = PyObject_GetAttrString(spec, "loader");
+    Py_DECREF(spec);
+    if (spec_loader == NULL) {
+        Py_DECREF(module_name);
+        return NULL;
+    }
+
     loader = _PyDict_GetItemWithError(module_globals, &_Py_ID(__loader__));
     if (loader == NULL) {
+        Py_DECREF(spec_loader);
+        Py_DECREF(module_name);
         return NULL;
     }
     Py_INCREF(loader);
-    module_name = _PyDict_GetItemWithError(module_globals, &_Py_ID(__name__));
-    if (!module_name) {
-        Py_DECREF(loader);
-        return NULL;
+
+    if (spec_loader != loader) {
+        int error = PyErr_WarnFormat(
+            PyExc_ImportWarning, 2,
+            "Module %U; __loader__ != __spec__.loader (%R != %R)",
+            module_name, spec_loader, loader);
+        if (error < 0) {
+            Py_DECREF(module_name);
+            Py_DECREF(spec_loader);
+            Py_DECREF(loader);
+            return NULL;
+        }
     }
-    Py_INCREF(module_name);
+    Py_DECREF(spec_loader);
 
     /* Make sure the loader implements the optional get_source() method. */
     (void)_PyObject_LookupAttr(loader, &_Py_ID(get_source), &get_source);