Add @solid_base (PEP 800)

Author: JelleZijlstra

CHANGELOG.md

@@ -1,3 +1,8 @@
+# Unreleased
+
+- Add the `@typing_extensions.solid_base` decorator, as specified
+  in PEP 800. Patch by Jelle Zijlstra.
+
 # Release 4.14.1 (July 4, 2025)
 
 - Fix usage of `typing_extensions.TypedDict` nested inside other types

doc/index.rst

@@ -748,6 +748,17 @@ Decorators
    improved, and ``typing_extensions`` backports these performance
    improvements.
 
+.. decorator:: solid_base
+
+   See :pep:`800`. A class decorator that marks a class as a "solid base", meaning that
+   child classes of the decorated class cannot inherit from other solid bases that are not
+   parent classes of the decorated class.
+
+   This helps type checkers to detect unreachable code and to understand when two types
+   can overlap.
+
+   .. versionadded:: 4.15.0
+
 Functions
 ~~~~~~~~~
 

src/test_typing_extensions.py

@@ -101,6 +101,7 @@
     reveal_type,
     runtime,
     runtime_checkable,
+    solid_base,
 )
 
 NoneType = type(None)
@@ -6669,6 +6670,18 @@ def cached(self): ...
         self.assertIs(True, Methods.cached.__final__)
 
 
+class SolidBaseTests(BaseTestCase):
+    def test_solid_base_unmodified(self):
+        class C: ...
+        self.assertIs(C, solid_base(C))
+
+    def test_dunder_solid_base(self):
+        @solid_base
+        class C: ...
+
+        self.assertIs(C.__solid_base__, True)
+
+
 class RevealTypeTests(BaseTestCase):
     def test_reveal_type(self):
         obj = object()

src/typing_extensions.py

@@ -315,6 +315,32 @@ class Other(Leaf):  # Error reported by type checker
         return f
 
 
+if hasattr(typing, "solid_base"):
+    solid_base = typing.solid_base
+else:
+    def solid_base(cls):
+        """This decorator marks a class a solid base.
+        Child classes of a solid base cannot inherit from other solid bases that are
+        not parent classes of the solid base.
+
+        For example:
+
+            @solid_base
+            class Solid1: pass
+
+            @solid_base
+            class Solid2: pass
+
+            class Solid3(Solid1, Solid2): pass  # Type checker error
+
+        Type checkers can use solid bases to detect unreachable code
+        and determine when two types can overlap.
+
+        See PEP 800."""
+        cls.__solid_base__ = True
+        return cls
+
+
 def IntVar(name):
     return typing.TypeVar(name)