Bugfix

mypy/fastparse.py

@@ -509,11 +509,12 @@ def fix_function_overloads(self, stmts: List[Statement]) -> List[Statement]:
                     and len(stmt.body[0].body) == 1
                     and isinstance(
                         stmt.body[0].body[0], (Decorator, OverloadedFuncDef))
-                    and infer_reachability_of_if_statement(
+                    and infer_reachability_of_if_statement(  # type: ignore[func-returns-value]
                         stmt, self.options
-                    ) is None  # type: ignore[func-returns-value]
+                    ) is None
                     and stmt.body[0].is_unreachable is False
                 ):
+                    current_overload = []
                     current_overload_name = stmt.body[0].body[0].name
                     last_if_stmt = stmt
                     last_if_overload = stmt.body[0].body[0]
@@ -526,6 +527,8 @@ def fix_function_overloads(self, stmts: List[Statement]) -> List[Statement]:
             ret.append(current_overload[0])
         elif len(current_overload) > 1:
             ret.append(OverloadedFuncDef(current_overload))
+        elif last_if_stmt is not None:
+            ret.append(last_if_stmt)
         return ret
 
     def in_method_scope(self) -> bool:

test-data/unit/check-overloading.test

@@ -5496,3 +5496,34 @@ def f2(g: Any) -> Any: ...
 reveal_type(f2(42))  # N: Revealed type is "__main__.A"
 reveal_type(f2("Hello"))  # N: Revealed type is "__main__.A" \
                           # E: Argument 1 to "f2" has incompatible type "str"; expected "int"
+
+[case testOverloadIfOldStyle]
+# flags: --always-false var_false --always-true var_true
+from typing import overload, Any
+
+class A: ...
+class B: ...
+
+var_true = True
+var_false = False
+
+if var_false:
+    @overload
+    def f1(g: int) -> A: ...
+    @overload
+    def f1(g: str) -> B: ...
+    def f1(g: Any) -> Any: ...
+elif var_true:
+    @overload
+    def f1(g: int) -> A: ...
+    @overload
+    def f1(g: str) -> B: ...
+    def f1(g: Any) -> Any: ...
+else:
+    @overload
+    def f1(g: int) -> A: ...
+    @overload
+    def f1(g: str) -> B: ...
+    def f1(g: Any) -> Any: ...
+reveal_type(f1(42))  # N: Revealed type is "__main__.A"
+reveal_type(f1("Hello"))  # N: Revealed type is "__main__.B"