Do not consider bare TypeVar not overlapping with None for reachabili…

…ty analysis (#18138)

Fixes #18126.

Simply allowing such intersection was insufficient: existing binder
logic widened the type to `T | None` after the `is None` check.

This PR extends the binder logic to prevent constructing a union type
when all conditional branches are reachable and contain no assignments:
checking `if isinstance(something, Something)` does not change the type
of `something` after the end of the `if` block.

mypy/binder.py

@@ -2,7 +2,7 @@
 
 from collections import defaultdict
 from contextlib import contextmanager
-from typing import DefaultDict, Iterator, List, Optional, Tuple, Union, cast
+from typing import DefaultDict, Iterator, List, NamedTuple, Optional, Tuple, Union
 from typing_extensions import TypeAlias as _TypeAlias
 
 from mypy.erasetype import remove_instance_last_known_values
@@ -30,6 +30,11 @@
 BindableExpression: _TypeAlias = Union[IndexExpr, MemberExpr, NameExpr]
 
 
+class CurrentType(NamedTuple):
+    type: Type
+    from_assignment: bool
+
+
 class Frame:
     """A Frame represents a specific point in the execution of a program.
     It carries information about the current types of expressions at
@@ -44,7 +49,7 @@ class Frame:
 
     def __init__(self, id: int, conditional_frame: bool = False) -> None:
         self.id = id
-        self.types: dict[Key, Type] = {}
+        self.types: dict[Key, CurrentType] = {}
         self.unreachable = False
         self.conditional_frame = conditional_frame
         self.suppress_unreachable_warnings = False
@@ -132,18 +137,18 @@ def push_frame(self, conditional_frame: bool = False) -> Frame:
         self.options_on_return.append([])
         return f
 
-    def _put(self, key: Key, type: Type, index: int = -1) -> None:
-        self.frames[index].types[key] = type
+    def _put(self, key: Key, type: Type, from_assignment: bool, index: int = -1) -> None:
+        self.frames[index].types[key] = CurrentType(type, from_assignment)
 
-    def _get(self, key: Key, index: int = -1) -> Type | None:
+    def _get(self, key: Key, index: int = -1) -> CurrentType | None:
         if index < 0:
             index += len(self.frames)
         for i in range(index, -1, -1):
             if key in self.frames[i].types:
                 return self.frames[i].types[key]
         return None
 
-    def put(self, expr: Expression, typ: Type) -> None:
+    def put(self, expr: Expression, typ: Type, *, from_assignment: bool = True) -> None:
         if not isinstance(expr, (IndexExpr, MemberExpr, NameExpr)):
             return
         if not literal(expr):
@@ -153,7 +158,7 @@ def put(self, expr: Expression, typ: Type) -> None:
         if key not in self.declarations:
             self.declarations[key] = get_declaration(expr)
             self._add_dependencies(key)
-        self._put(key, typ)
+        self._put(key, typ, from_assignment)
 
     def unreachable(self) -> None:
         self.frames[-1].unreachable = True
@@ -164,7 +169,10 @@ def suppress_unreachable_warnings(self) -> None:
     def get(self, expr: Expression) -> Type | None:
         key = literal_hash(expr)
         assert key is not None, "Internal error: binder tried to get non-literal"
-        return self._get(key)
+        found = self._get(key)
+        if found is None:
+            return None
+        return found.type
 
     def is_unreachable(self) -> bool:
         # TODO: Copy the value of unreachable into new frames to avoid
@@ -193,7 +201,7 @@ def update_from_options(self, frames: list[Frame]) -> bool:
         If a key is declared as AnyType, only update it if all the
         options are the same.
         """
-
+        all_reachable = all(not f.unreachable for f in frames)
         frames = [f for f in frames if not f.unreachable]
         changed = False
         keys = {key for f in frames for key in f.types}
@@ -207,17 +215,30 @@ def update_from_options(self, frames: list[Frame]) -> bool:
                 # know anything about key in at least one possible frame.
                 continue
 
-            type = resulting_values[0]
-            assert type is not None
+            if all_reachable and all(
+                x is not None and not x.from_assignment for x in resulting_values
+            ):
+                # Do not synthesize a new type if we encountered a conditional block
+                # (if, while or match-case) without assignments.
+                # See check-isinstance.test::testNoneCheckDoesNotMakeTypeVarOptional
+                # This is a safe assumption: the fact that we checked something with `is`
+                # or `isinstance` does not change the type of the value.
+                continue
+
+            current_type = resulting_values[0]
+            assert current_type is not None
+            type = current_type.type
             declaration_type = get_proper_type(self.declarations.get(key))
             if isinstance(declaration_type, AnyType):
                 # At this point resulting values can't contain None, see continue above
-                if not all(is_same_type(type, cast(Type, t)) for t in resulting_values[1:]):
+                if not all(
+                    t is not None and is_same_type(type, t.type) for t in resulting_values[1:]
+                ):
                     type = AnyType(TypeOfAny.from_another_any, source_any=declaration_type)
             else:
                 for other in resulting_values[1:]:
                     assert other is not None
-                    type = join_simple(self.declarations[key], type, other)
+                    type = join_simple(self.declarations[key], type, other.type)
                     # Try simplifying resulting type for unions involving variadic tuples.
                     # Technically, everything is still valid without this step, but if we do
                     # not do this, this may create long unions after exiting an if check like:
@@ -236,8 +257,8 @@ def update_from_options(self, frames: list[Frame]) -> bool:
                         )
                         if simplified == self.declarations[key]:
                             type = simplified
-            if current_value is None or not is_same_type(type, current_value):
-                self._put(key, type)
+            if current_value is None or not is_same_type(type, current_value[0]):
+                self._put(key, type, from_assignment=True)
                 changed = True
 
         self.frames[-1].unreachable = not frames
@@ -374,7 +395,9 @@ def most_recent_enclosing_type(self, expr: BindableExpression, type: Type) -> Ty
         key = literal_hash(expr)
         assert key is not None
         enclosers = [get_declaration(expr)] + [
-            f.types[key] for f in self.frames if key in f.types and is_subtype(type, f.types[key])
+            f.types[key].type
+            for f in self.frames
+            if key in f.types and is_subtype(type, f.types[key][0])
         ]
         return enclosers[-1]
 

mypy/checker.py

@@ -4725,11 +4725,11 @@ def visit_if_stmt(self, s: IfStmt) -> None:
 
                 # XXX Issue a warning if condition is always False?
                 with self.binder.frame_context(can_skip=True, fall_through=2):
-                    self.push_type_map(if_map)
+                    self.push_type_map(if_map, from_assignment=False)
                     self.accept(b)
 
                 # XXX Issue a warning if condition is always True?
-                self.push_type_map(else_map)
+                self.push_type_map(else_map, from_assignment=False)
 
             with self.binder.frame_context(can_skip=False, fall_through=2):
                 if s.else_body:
@@ -5310,18 +5310,21 @@ def visit_match_stmt(self, s: MatchStmt) -> None:
                     if b.is_unreachable or isinstance(
                         get_proper_type(pattern_type.type), UninhabitedType
                     ):
-                        self.push_type_map(None)
+                        self.push_type_map(None, from_assignment=False)
                         else_map: TypeMap = {}
                     else:
                         pattern_map, else_map = conditional_types_to_typemaps(
                             named_subject, pattern_type.type, pattern_type.rest_type
                         )
                         self.remove_capture_conflicts(pattern_type.captures, inferred_types)
-                        self.push_type_map(pattern_map)
+                        self.push_type_map(pattern_map, from_assignment=False)
                         if pattern_map:
                             for expr, typ in pattern_map.items():
-                                self.push_type_map(self._get_recursive_sub_patterns_map(expr, typ))
-                        self.push_type_map(pattern_type.captures)
+                                self.push_type_map(
+                                    self._get_recursive_sub_patterns_map(expr, typ),
+                                    from_assignment=False,
+                                )
+                        self.push_type_map(pattern_type.captures, from_assignment=False)
                     if g is not None:
                         with self.binder.frame_context(can_skip=False, fall_through=3):
                             gt = get_proper_type(self.expr_checker.accept(g))
@@ -5347,11 +5350,11 @@ def visit_match_stmt(self, s: MatchStmt) -> None:
                                                 continue
                                             type_map[named_subject] = type_map[expr]
 
-                            self.push_type_map(guard_map)
+                            self.push_type_map(guard_map, from_assignment=False)
                             self.accept(b)
                     else:
                         self.accept(b)
-                self.push_type_map(else_map)
+                self.push_type_map(else_map, from_assignment=False)
 
             # This is needed due to a quirk in frame_context. Without it types will stay narrowed
             # after the match.
@@ -7372,12 +7375,12 @@ def iterable_item_type(
     def function_type(self, func: FuncBase) -> FunctionLike:
         return function_type(func, self.named_type("builtins.function"))
 
-    def push_type_map(self, type_map: TypeMap) -> None:
+    def push_type_map(self, type_map: TypeMap, *, from_assignment: bool = True) -> None:
         if type_map is None:
             self.binder.unreachable()
         else:
             for expr, type in type_map.items():
-                self.binder.put(expr, type)
+                self.binder.put(expr, type, from_assignment=from_assignment)
 
     def infer_issubclass_maps(self, node: CallExpr, expr: Expression) -> tuple[TypeMap, TypeMap]:
         """Infer type restrictions for an expression in issubclass call."""
@@ -7750,9 +7753,7 @@ def conditional_types(
         ) and is_proper_subtype(current_type, proposed_type, ignore_promotions=True):
             # Expression is always of one of the types in proposed_type_ranges
             return default, UninhabitedType()
-        elif not is_overlapping_types(
-            current_type, proposed_type, prohibit_none_typevar_overlap=True, ignore_promotions=True
-        ):
+        elif not is_overlapping_types(current_type, proposed_type, ignore_promotions=True):
             # Expression is never of any type in proposed_type_ranges
             return UninhabitedType(), default
         else:

test-data/unit/check-enum.test

@@ -815,7 +815,7 @@ elif x is Foo.C:
     reveal_type(x)  # N: Revealed type is "Literal[__main__.Foo.C]"
 else:
     reveal_type(x)  # No output here: this branch is unreachable
-reveal_type(x) # N: Revealed type is "__main__.Foo"
+reveal_type(x) # N: Revealed type is "Union[Literal[__main__.Foo.A], Literal[__main__.Foo.B], Literal[__main__.Foo.C]]"
 
 if Foo.A is x:
     reveal_type(x)  # N: Revealed type is "Literal[__main__.Foo.A]"
@@ -825,7 +825,7 @@ elif Foo.C is x:
     reveal_type(x)  # N: Revealed type is "Literal[__main__.Foo.C]"
 else:
     reveal_type(x)  # No output here: this branch is unreachable
-reveal_type(x) # N: Revealed type is "__main__.Foo"
+reveal_type(x) # N: Revealed type is "Union[Literal[__main__.Foo.A], Literal[__main__.Foo.B], Literal[__main__.Foo.C]]"
 
 y: Foo
 if y is Foo.A:

test-data/unit/check-isinstance.test

@@ -2207,23 +2207,24 @@ def foo2(x: Optional[str]) -> None:
         reveal_type(x)      # N: Revealed type is "builtins.str"
 [builtins fixtures/isinstance.pyi]
 
-[case testNoneCheckDoesNotNarrowWhenUsingTypeVars]
-
-# Note: this test (and the following one) are testing checker.conditional_type_map:
-# if you set the 'prohibit_none_typevar_overlap' keyword argument to False when calling
-# 'is_overlapping_types', the binder will incorrectly infer that 'out' has a type of
-# Union[T, None] after the if statement.
-
+[case testNoneCheckDoesNotMakeTypeVarOptional]
 from typing import TypeVar
 
 T = TypeVar('T')
 
-def foo(x: T) -> T:
+def foo_if(x: T) -> T:
     out = None
     out = x
     if out is None:
         pass
     return out
+
+def foo_while(x: T) -> T:
+    out = None
+    out = x
+    while out is None:
+        pass
+    return out
 [builtins fixtures/isinstance.pyi]
 
 [case testNoneCheckDoesNotNarrowWhenUsingTypeVarsNoStrictOptional]

test-data/unit/check-narrowing.test

@@ -2333,3 +2333,22 @@ def f(x: C) -> None:
 
 f(C(5))
 [builtins fixtures/primitives.pyi]
+
+[case testNarrowingTypeVarNone]
+# flags: --warn-unreachable
+
+# https://github.com/python/mypy/issues/18126
+from typing import TypeVar
+
+T = TypeVar("T")
+
+def fn_if(arg: T) -> None:
+    if arg is None:
+        return None
+    return None
+
+def fn_while(arg: T) -> None:
+    while arg is None:
+        return None
+    return None
+[builtins fixtures/primitives.pyi]

test-data/unit/check-python310.test

@@ -2409,3 +2409,33 @@ def f(x: T) -> None:
         case _:
             accept_seq_int(x)  # E: Argument 1 to "accept_seq_int" has incompatible type "T"; expected "Sequence[int]"
 [builtins fixtures/tuple.pyi]
+
+[case testNarrowingTypeVarMatch]
+# flags: --warn-unreachable
+
+# https://github.com/python/mypy/issues/18126
+from typing import TypeVar
+
+T = TypeVar("T")
+
+def fn_case(arg: T) -> None:
+    match arg:
+        case None:
+            return None
+    return None
+[builtins fixtures/primitives.pyi]
+
+[case testNoneCheckDoesNotMakeTypeVarOptionalMatch]
+from typing import TypeVar
+
+T = TypeVar('T')
+
+def foo(x: T) -> T:
+    out = None
+    out = x
+    match out:
+        case None:
+            pass
+    return out
+
+[builtins fixtures/isinstance.pyi]

test-data/unit/check-type-promotion.test

@@ -91,7 +91,7 @@ else:
     reveal_type(x)  # N: Revealed type is "builtins.complex"
 
 # Note we make type precise, since type promotions are involved
-reveal_type(x)  # N: Revealed type is "Union[builtins.complex, builtins.int, builtins.float]"
+reveal_type(x)  # N: Revealed type is "builtins.complex"
 [builtins fixtures/primitives.pyi]
 
 [case testIntersectionUsingPromotion3]
@@ -127,7 +127,7 @@ if isinstance(x, int):
     reveal_type(x)  # N: Revealed type is "builtins.int"
 else:
     reveal_type(x)  # N: Revealed type is "Union[builtins.float, builtins.complex]"
-reveal_type(x)  # N: Revealed type is "Union[builtins.int, builtins.float, builtins.complex]"
+reveal_type(x)  # N: Revealed type is "Union[builtins.float, builtins.complex]"
 [builtins fixtures/primitives.pyi]
 
 [case testIntersectionUsingPromotion6]
@@ -139,7 +139,7 @@ if isinstance(x, int):
     reveal_type(x)  # N: Revealed type is "builtins.int"
 else:
     reveal_type(x)  # N: Revealed type is "Union[builtins.str, builtins.complex]"
-reveal_type(x)  # N: Revealed type is "Union[builtins.str, builtins.int, builtins.complex]"
+reveal_type(x)  # N: Revealed type is "Union[builtins.str, builtins.complex]"
 [builtins fixtures/primitives.pyi]
 
 [case testIntersectionUsingPromotion7]