Fix #10280: autodoc_docstring_signature generates needless return typehint

Basically, autodoc suppresses return value typehint for class
constructors.  But it was unexpectedly shown if
`autodoc_docstring_signature` is enabled and docstring has multiple
signatures.

CHANGES

@@ -67,6 +67,8 @@ Bugs fixed
 
 * #10279: autodoc: Default values for keyword only arguments in overloaded
   functions are rendered as a string literal
+* #10280: autodoc: :confval:`autodoc_docstring_signature` unexpectedly generates
+  return value typehint for constructors if docstring has multiple signatures
 * #10214: html: invalid language tag was generated if :confval:`language`
   contains a country code (ex. zh_CN)
 * #10236: html search: objects are duplicated in search result

sphinx/ext/autodoc/__init__.py

@@ -1580,6 +1580,20 @@ class ClassDocumenter(DocstringSignatureMixin, ModuleLevelDocumenter):  # type:
 
         return stringify_signature(sig, show_return_annotation=False, **kwargs)
 
+    def _find_signature(self) -> Tuple[str, str]:
+        result = super()._find_signature()
+        if result is not None:
+            # Strip a return value from signature of constructor in docstring (first entry)
+            result = (result[0], None)
+
+        for i, sig in enumerate(self._signatures):
+            if sig.endswith(' -> None'):
+                # Strip a return value from signatures of constructor in docstring (subsequent
+                # entries)
+                self._signatures[i] = sig[:-8]
+
+        return result
+
     def format_signature(self, **kwargs: Any) -> str:
         if self.doc_as_attr:
             return ''

tests/roots/test-ext-autodoc/target/docstring_signature.py

@@ -22,10 +22,12 @@ class D:
 class E:
     def __init__(self):
         """E(foo: int, bar: int, baz: int) -> None \\
-        E(foo: str, bar: str, baz: str) -> None"""
+        E(foo: str, bar: str, baz: str) -> None \\
+        E(foo: float, bar: float, baz: float)"""
 
 
 class F:
     def __init__(self):
         """F(foo: int, bar: int, baz: int) -> None
-        F(foo: str, bar: str, baz: str) -> None"""
+        F(foo: str, bar: str, baz: str) -> None
+        F(foo: float, bar: float, baz: float)"""

tests/test_ext_autodoc_configs.py

@@ -467,13 +467,15 @@ def test_autoclass_content_and_docstring_signature_init(app):
         '   :module: target.docstring_signature',
         '',
         '',
-        '.. py:class:: E(foo: int, bar: int, baz: int) -> None',
+        '.. py:class:: E(foo: int, bar: int, baz: int)',
-        '              E(foo: str, bar: str, baz: str) -> None',
+        '              E(foo: str, bar: str, baz: str)',
+        '              E(foo: float, bar: float, baz: float)',
         '   :module: target.docstring_signature',
         '',
         '',
-        '.. py:class:: F(foo: int, bar: int, baz: int) -> None',
+        '.. py:class:: F(foo: int, bar: int, baz: int)',
-        '              F(foo: str, bar: str, baz: str) -> None',
+        '              F(foo: str, bar: str, baz: str)',
+        '              F(foo: float, bar: float, baz: float)',
         '   :module: target.docstring_signature',
         '',
     ]
@@ -510,13 +512,15 @@ def test_autoclass_content_and_docstring_signature_both(app):
         '   :module: target.docstring_signature',
         '',
         '',
-        '.. py:class:: E(foo: int, bar: int, baz: int) -> None',
+        '.. py:class:: E(foo: int, bar: int, baz: int)',
-        '              E(foo: str, bar: str, baz: str) -> None',
+        '              E(foo: str, bar: str, baz: str)',
+        '              E(foo: float, bar: float, baz: float)',
         '   :module: target.docstring_signature',
         '',
         '',
-        '.. py:class:: F(foo: int, bar: int, baz: int) -> None',
+        '.. py:class:: F(foo: int, bar: int, baz: int)',
-        '              F(foo: str, bar: str, baz: str) -> None',
+        '              F(foo: str, bar: str, baz: str)',
+        '              F(foo: float, bar: float, baz: float)',
         '   :module: target.docstring_signature',
         '',
     ]