Allow binder to understand assignment via def (#13508)

Fixes #2569

Author: hauntsaninja

mypy/checker.py

@@ -972,7 +972,9 @@ def _visit_func_def(self, defn: FuncDef) -> None:
                         # Trying to redefine something like partial empty list as function.
                         self.fail(message_registry.INCOMPATIBLE_REDEFINITION, defn)
                 else:
-                    # TODO: Update conditional type binder.
+                    name_expr = NameExpr(defn.name)
+                    name_expr.node = defn.original_def
+                    self.binder.assign_type(name_expr, new_type, orig_type)
                     self.check_subtype(
                         new_type,
                         orig_type,

test-data/unit/check-narrowing.test

@@ -1249,3 +1249,14 @@ def two_type_vars(x: Union[str, Dict[str, int], Dict[bool, object], int]) -> Non
     else:
         reveal_type(x)  # N: Revealed type is "builtins.int"
 [builtins fixtures/dict.pyi]
+
+
+[case testNarrowingWithDef]
+from typing import Callable, Optional
+
+def g() -> None:
+    foo: Optional[Callable[[], None]] = None
+    if foo is None:
+        def foo(): ...
+    foo()
+[builtins fixtures/dict.pyi]