Fix #9121: py domain: duplicated warning for canonical and alias

A duplicated warning is emitted when both canonical and its alias
objects are defined on the same document. But it should not be emitted
because they're the same object, not conflicted.

CHANGES

@@ -23,6 +23,8 @@ Features added
 * #9098: html: copy-range protection for doctests doesn't work in Safari
 * #9103: LaTeX: imgconverter: conversion runs even if not needed
 * #8127: py domain: Ellipsis in info-field-list causes nit-picky warning
+* #9121: py domain: duplicated warning is emitted when both canonical and its
+  alias objects are defined on the document
 * #9023: More CSS classes on domain descriptions, see :ref:`nodes` for details.
 
 Bugs fixed

sphinx/domains/python.py

@@ -68,7 +68,7 @@ class ObjectEntry(NamedTuple):
     docname: str
     node_id: str
     objtype: str
-    canonical: bool
+    aliased: bool
 
 
 class ModuleEntry(NamedTuple):
@@ -505,7 +505,7 @@ class PyObject(ObjectDescription[Tuple[str, str]]):
 
         canonical_name = self.options.get('canonical')
         if canonical_name:
-            domain.note_object(canonical_name, self.objtype, node_id, canonical=True,
+            domain.note_object(canonical_name, self.objtype, node_id, aliased=True,
                                location=signode)
 
         if 'noindexentry' not in self.options:
@@ -1138,17 +1138,25 @@ class PythonDomain(Domain):
         return self.data.setdefault('objects', {})  # fullname -> ObjectEntry
 
     def note_object(self, name: str, objtype: str, node_id: str,
-                    canonical: bool = False, location: Any = None) -> None:
+                    aliased: bool = False, location: Any = None) -> None:
         """Note a python object for cross reference.
 
         .. versionadded:: 2.1
         """
         if name in self.objects:
             other = self.objects[name]
+            if other.aliased and aliased is False:
+                # The original definition found. Override it!
+                pass
+            elif other.aliased is False and aliased:
+                # The original definition is already registered.
+                return
+            else:
+                # duplicated
                 logger.warning(__('duplicate object description of %s, '
                                   'other instance in %s, use :noindex: for one of them'),
                                name, other.docname, location=location)
-        self.objects[name] = ObjectEntry(self.env.docname, node_id, objtype, canonical)
+        self.objects[name] = ObjectEntry(self.env.docname, node_id, objtype, aliased)
 
     @property
     def modules(self) -> Dict[str, ModuleEntry]:
@@ -1326,8 +1334,8 @@ class PythonDomain(Domain):
             yield (modname, modname, 'module', mod.docname, mod.node_id, 0)
         for refname, obj in self.objects.items():
             if obj.objtype != 'module':  # modules are already handled
-                if obj.canonical:
+                if obj.aliased:
-                    # canonical names are not full-text searchable.
+                    # aliased names are not full-text searchable.
                     yield (refname, refname, obj.objtype, obj.docname, obj.node_id, -1)
                 else:
                     yield (refname, refname, obj.objtype, obj.docname, obj.node_id, 1)

tests/test_domain_py.py

@@ -870,6 +870,39 @@ def test_canonical(app):
     assert domain.objects['_io.StringIO'] == ('index', 'io.StringIO', 'class', True)
 
 
+def test_canonical_definition_overrides(app, warning):
+    text = (".. py:class:: io.StringIO\n"
+            "   :canonical: _io.StringIO\n"
+            ".. py:class:: _io.StringIO\n")
+    restructuredtext.parse(app, text)
+    assert warning.getvalue() == ""
+
+    domain = app.env.get_domain('py')
+    assert domain.objects['_io.StringIO'] == ('index', 'id0', 'class', False)
+
+
+def test_canonical_definition_skip(app, warning):
+    text = (".. py:class:: _io.StringIO\n"
+            ".. py:class:: io.StringIO\n"
+            "   :canonical: _io.StringIO\n")
+
+    restructuredtext.parse(app, text)
+    assert warning.getvalue() == ""
+
+    domain = app.env.get_domain('py')
+    assert domain.objects['_io.StringIO'] == ('index', 'io.StringIO', 'class', False)
+
+
+def test_canonical_duplicated(app, warning):
+    text = (".. py:class:: mypackage.StringIO\n"
+            "   :canonical: _io.StringIO\n"
+            ".. py:class:: io.StringIO\n"
+            "   :canonical: _io.StringIO\n")
+
+    restructuredtext.parse(app, text)
+    assert warning.getvalue() != ""
+
+
 def test_info_field_list(app):
     text = (".. py:module:: example\n"
             ".. py:class:: Class\n"