Fix crash with assignment to variable guarded with TypeGuard (#10683)

This is a quick fix to unblock the 0.910 release.

The type guard type can be unrelated to the original type, so we
shouldn't join it. I'm not sure whether this is right from first
principles, but it seems to address the issue.

Fixes #10671.

mypy/binder.py

@@ -5,7 +5,7 @@
 from typing_extensions import DefaultDict
 
 from mypy.types import (
-    Type, AnyType, PartialType, UnionType, TypeOfAny, NoneType, get_proper_type
+    Type, AnyType, PartialType, UnionType, TypeOfAny, NoneType, TypeGuardType, get_proper_type
 )
 from mypy.subtypes import is_subtype
 from mypy.join import join_simple
@@ -202,7 +202,9 @@ def update_from_options(self, frames: List[Frame]) -> bool:
             else:
                 for other in resulting_values[1:]:
                     assert other is not None
-                    type = join_simple(self.declarations[key], type, other)
+                    # Ignore the error about using get_proper_type().
+                    if not isinstance(other, TypeGuardType):  # type: ignore[misc]
+                        type = join_simple(self.declarations[key], type, other)
             if current_value is None or not is_same_type(type, current_value):
                 self._put(key, type)
                 changed = True

test-data/unit/check-typeguard.test

@@ -82,6 +82,7 @@ def is_str_list(a: List[object]) -> TypeGuard[List[str]]: pass
 def main(a: List[object]):
     if is_str_list(a):
         reveal_type(a)  # N: Revealed type is "builtins.list[builtins.str]"
+    reveal_type(a)  # N: Revealed type is "builtins.list[builtins.object]"
 [builtins fixtures/tuple.pyi]
 
 [case testTypeGuardUnionIn]
@@ -91,6 +92,7 @@ def is_foo(a: Union[int, str]) -> TypeGuard[str]: pass
 def main(a: Union[str, int]) -> None:
     if is_foo(a):
         reveal_type(a)  # N: Revealed type is "builtins.str"
+    reveal_type(a)  # N: Revealed type is "Union[builtins.str, builtins.int]"
 [builtins fixtures/tuple.pyi]
 
 [case testTypeGuardUnionOut]
@@ -315,3 +317,50 @@ def coverage(obj: Any) -> bool:
         return True
     return False
 [builtins fixtures/classmethod.pyi]
+
+[case testAssignToTypeGuardedVariable1]
+from typing_extensions import TypeGuard
+
+class A: pass
+class B(A): pass
+
+def guard(a: A) -> TypeGuard[B]:
+    pass
+
+a = A()
+if not guard(a):
+    a = A()
+[builtins fixtures/tuple.pyi]
+
+[case testAssignToTypeGuardedVariable2]
+from typing_extensions import TypeGuard
+
+class A: pass
+class B: pass
+
+def guard(a: A) -> TypeGuard[B]:
+    pass
+
+a = A()
+if not guard(a):
+    a = A()
+[builtins fixtures/tuple.pyi]
+
+[case testAssignToTypeGuardedVariable3]
+from typing_extensions import TypeGuard
+
+class A: pass
+class B: pass
+
+def guard(a: A) -> TypeGuard[B]:
+    pass
+
+a = A()
+if guard(a):
+    reveal_type(a)  # N: Revealed type is "__main__.B"
+    a = B()  # E: Incompatible types in assignment (expression has type "B", variable has type "A")
+    reveal_type(a)  # N: Revealed type is "__main__.B"
+    a = A()
+    reveal_type(a)  # N: Revealed type is "__main__.A"
+reveal_type(a)  # N: Revealed type is "__main__.A"
+[builtins fixtures/tuple.pyi]