Fix #7850: autodoc: KeyError is raised for invalid mark up

CHANGES

@@ -19,6 +19,8 @@ Bugs fixed
 * #7844: autodoc: Failed to detect module when relative module name given
 * #7856: autodoc: AttributeError is raised when non-class object is given to
   the autoclass directive
+* #7850: autodoc: KeyError is raised for invalid mark up when autodoc_typehints
+  is 'description'
 
 Testing
 --------

sphinx/ext/autodoc/typehints.py

@@ -46,11 +46,16 @@ def merge_typehints(app: Sphinx, domain: str, objtype: str, contentnode: Element
     if objtype == 'class' and app.config.autoclass_content not in ('init', 'both'):
         return
 
-    signature = cast(addnodes.desc_signature, contentnode.parent[0])
-    if signature['module']:
-        fullname = '.'.join([signature['module'], signature['fullname']])
-    else:
-        fullname = signature['fullname']
+    try:
+        signature = cast(addnodes.desc_signature, contentnode.parent[0])
+        if signature['module']:
+            fullname = '.'.join([signature['module'], signature['fullname']])
+        else:
+            fullname = signature['fullname']
+    except KeyError:
+        # signature node does not have valid context info for the target object
+        return
+
     annotations = app.env.temp_data.get('annotations', {})
     if annotations.get(fullname, {}):
         field_lists = [n for n in contentnode if isinstance(n, nodes.field_list)]

tests/test_ext_autodoc_configs.py

@@ -13,6 +13,8 @@ import sys
 
 import pytest
 
+from sphinx.testing import restructuredtext
+
 from test_ext_autodoc import do_autodoc
 
 IS_PYPY = platform.python_implementation() == 'PyPy'
@@ -633,6 +635,12 @@ def test_autodoc_typehints_description(app):
             in context)
 
 
+@pytest.mark.sphinx('text', testroot='ext-autodoc',
+                    confoverrides={'autodoc_typehints': "description"})
+def test_autodoc_typehints_description_for_invalid_node(app):
+    text = ".. py:function:: hello; world"
+    restructuredtext.parse(app, text)  # raises no error
+
 
 @pytest.mark.sphinx('html', testroot='ext-autodoc')
 def test_autodoc_default_options(app):