Test import from typing_extensions

Also, mark the import from typing as Python 3.12 only. And change the
message a bit.

mypy/messages.py

@@ -1496,7 +1496,7 @@ def cant_assign_to_classvar(self, name: str, context: Context) -> None:
     def no_overridable_method(self, name: str, context: Context) -> None:
         self.fail(
             f'Method "{name}" is marked as an override, '
-            "but no base method with this name was found",
+            "but no base method was found with this name",
             context,
         )
 

test-data/unit/check-functions.test

@@ -2726,12 +2726,13 @@ g: Callable[[Union[Sequence[TI], Sequence[TS]]], None]
 f = g
 
 [case explicitOverride]
+# flags: --python-version 3.12
 from typing import override
 
 class A:
     def f(self, x: int) -> str: pass
     @override
-    def g(self, x: int) -> str: pass  # E: Method "g" is marked as an override, but no base method with this name was found
+    def g(self, x: int) -> str: pass  # E: Method "g" is marked as an override, but no base method was found with this name
 
 class B(A):
     @override
@@ -2755,6 +2756,7 @@ class F(E):
 [builtins fixtures/tuple.pyi]
 
 [case explicitOverrideStaticmethod]
+# flags: --python-version 3.12
 from typing import override
 
 class A:
@@ -2767,15 +2769,15 @@ class B(A):
     def f(x: int) -> str: pass
     @override
     @staticmethod
-    def g(x: int) -> str: pass  # E: Method "g" is marked as an override, but no base method with this name was found
+    def g(x: int) -> str: pass  # E: Method "g" is marked as an override, but no base method was found with this name
 
 class C(A):  # inverted order of decorators
     @override
     @staticmethod
     def f(x: int) -> str: pass
     @override
     @staticmethod
-    def g(x: int) -> str: pass  # E: Method "g" is marked as an override, but no base method with this name was found
+    def g(x: int) -> str: pass  # E: Method "g" is marked as an override, but no base method was found with this name
 
 class D(A):
     @staticmethod
@@ -2787,6 +2789,7 @@ class D(A):
 [builtins fixtures/callable.pyi]
 
 [case explicitOverrideClassmethod]
+# flags: --python-version 3.12
 from typing import override
 
 class A:
@@ -2799,15 +2802,15 @@ class B(A):
     def f(cls, x: int) -> str: pass
     @override
     @classmethod
-    def g(cls, x: int) -> str: pass  # E: Method "g" is marked as an override, but no base method with this name was found
+    def g(cls, x: int) -> str: pass  # E: Method "g" is marked as an override, but no base method was found with this name
 
 class C(A):  # inverted order of decorators
     @override
     @classmethod
     def f(cls, x: int) -> str: pass
     @override
     @classmethod
-    def g(cls, x: int) -> str: pass  # E: Method "g" is marked as an override, but no base method with this name was found
+    def g(cls, x: int) -> str: pass  # E: Method "g" is marked as an override, but no base method was found with this name
 
 class D(A):
     @classmethod
@@ -2819,6 +2822,7 @@ class D(A):
 [builtins fixtures/callable.pyi]
 
 [case explicitOverrideProperty]
+# flags: --python-version 3.12
 from typing import override
 
 class A:
@@ -2831,15 +2835,15 @@ class B(A):
     def f(self) -> str: pass
     @override
     @property
-    def g(self) -> str: pass  # E: Method "g" is marked as an override, but no base method with this name was found
+    def g(self) -> str: pass  # E: Method "g" is marked as an override, but no base method was found with this name
 
 class C(A):  # inverted order of decorators
     @override
     @property
     def f(self) -> str: pass
     @override
     @property
-    def g(self) -> str: pass  # E: Method "g" is marked as an override, but no base method with this name was found
+    def g(self) -> str: pass  # E: Method "g" is marked as an override, but no base method was found with this name
 
 class D(A):
     @property
@@ -2849,6 +2853,7 @@ class D(A):
 [typing fixtures/typing-full.pyi]
 
 [case invalidExplicitOverride]
+# flags: --python-version 3.12
 from typing import override
 
 @override  # E: "override" used with a non-method
@@ -2864,6 +2869,7 @@ def g() -> None:
 [builtins fixtures/tuple.pyi]
 
 [case explicitOverrideSpecialMethods]
+# flags: --python-version 3.12
 from typing import override
 
 class A:
@@ -2878,3 +2884,15 @@ class C:
     def __init__(self, a: int) -> None: pass
 [typing fixtures/typing-full.pyi]
 [builtins fixtures/tuple.pyi]
+
+[case explicitOverrideFromExtensions]
+from typing_extensions import override
+
+class A:
+    def f(self, x: int) -> str: pass
+
+class B(A):
+    @override
+    def f2(self, x: int) -> str: pass  # E: Method "f2" is marked as an override, but no base method was found with this name
+[typing fixtures/typing-full.pyi]
+[builtins fixtures/tuple.pyi]

test-data/unit/lib-stub/typing_extensions.pyi

@@ -74,4 +74,6 @@ def dataclass_transform(
     **kwargs: Any,
 ) -> Callable[[T], T]: ...
 
+def override(__arg: _T) -> _T: ...
+
 _FutureFeatureFixture = 0