Don't flag intentionally empty generators unreachable

mypy/checker.py

@@ -132,6 +132,7 @@
     Var,
     WhileStmt,
     WithStmt,
+    YieldExpr,
     is_final_node,
 )
 from mypy.options import Options
@@ -1063,6 +1064,20 @@ def enter_attribute_inference_context(self) -> Iterator[None]:
         yield None
         self.inferred_attribute_types = old_types
 
+    def _is_empty_generator(self, func: FuncItem) -> bool:
+        """
+        Checks whether a function's body is 'return; yield' (the yield being added only
+        to promote the function into a generator).
+        """
+        return (
+            len(body := func.body.body) == 2
+            and isinstance(ret_stmt := body[0], ReturnStmt)
+            and (ret_stmt.expr is None or is_literal_none(ret_stmt.expr))
+            and isinstance(expr_stmt := body[1], ExpressionStmt)
+            and isinstance(yield_expr := expr_stmt.expr, YieldExpr)
+            and (yield_expr.expr is None or is_literal_none(yield_expr.expr))
+        )
+
     def check_func_def(
         self, defn: FuncItem, typ: CallableType, name: str | None, allow_empty: bool = False
     ) -> None:
@@ -1240,7 +1255,7 @@ def check_func_def(
                     # have no good way of doing this.
                     #
                     # TODO: Find a way of working around this limitation
-                    if len(expanded) >= 2:
+                    if len(expanded) >= 2 or self._is_empty_generator(item):
                         self.binder.suppress_unreachable_warnings()
                     self.accept(item.body)
                 unreachable = self.binder.is_unreachable()

test-data/unit/check-unreachable-code.test

@@ -1446,3 +1446,19 @@ def f() -> None:
     Foo()['a'] = 'a'
     x = 0 # This should not be reported as unreachable
 [builtins fixtures/exception.pyi]
+
+[case testIntentionallyEmptyGenerator]
+# flags: --warn-unreachable
+from typing import Generator
+
+def f() -> Generator[None, None, None]:
+    return
+    yield
+
+[case testIntentionallyEmptyGenerator_None]
+# flags: --warn-unreachable
+from typing import Generator
+
+def f() -> Generator[None, None, None]:
+    return None
+    yield None