Do not repor trivial narrowing as a change in binder (#20862)

Fixes #20816

This makes `--allow-redefinition-new` ~10% faster (or maybe more if you
have too many loops in your code). The idea is quite simple: there is no
difference between no narrowing, and narrowing to type that is a
variable declared type. This doesn't really make a difference for
regular mode, IIUC because there is a "fall through", i.e.
zero-iterations case, but this doesn't work with `bind_all = True`. As a
result we used to accept _every_ loop with an assignment in it at lest
twice with `--allow-redefinition-new` (which includes assignment to
iteration variable in for-loop).

Note that `declaration_type` may be `None` if there is a partial type,
but we already track partial types in accept loop separately, so we
don't need to report them as changed here as well.

Author: ilevkivskyi

mypy/binder.py

@@ -392,7 +392,22 @@ def update_from_options(self, frames: list[Frame]) -> bool:
                             type = simplified
             if current_value is None or not is_same_type(type, current_value.type):
                 self._put(key, type, from_assignment=True)
-                changed = True
+                if current_value is not None or extract_var_from_literal_hash(key) is None:
+                    # We definitely learned something new
+                    changed = True
+                else:
+                    # If there is no current value compare with the declaration. This prevents
+                    # reporting false changes in cases like this:
+                    #     x: int
+                    #     if foo():
+                    #         x = 1
+                    #     else:
+                    #         x = 2
+                    # We check partial types and widening in accept_loop() separately, so
+                    # this should be safe.
+                    changed = declaration_type is not None and not is_same_type(
+                        type, declaration_type
+                    )
 
         self.frames[-1].unreachable = not frames