Fix crash when using member type aliases in runtime contexts (#13602)

This pull request:

- Fixes #10357
- Fixes #9908

Currently, checkmember.py attempts handling only type aliases with Instance targets 
and ignores type aliases with a special form target such as `Union[...]`, `Literal[...]`, 
or `Callable[...]`, causing either a crash or odd runtime behavior.

This diff replaces that logic (the `instance_alias_type` function) with the more 
general-purpose `alias_type_in_runtime_context` function found in checkexpr.py.

I'm not actually 100% sure if the latter is a perfect substitute for the former -- the 
two functions seem to handle Instance type aliases a little differently. But I think this 
is probably fine: the long-term benefits of consolidating mypy's logic is probably 
worth some short-term risk.

mypy/checker.py

@@ -2309,9 +2309,7 @@ def determine_type_of_member(self, sym: SymbolTableNode) -> Type | None:
             with self.msg.filter_errors():
                 # Suppress any errors, they will be given when analyzing the corresponding node.
                 # Here we may have incorrect options and location context.
-                return self.expr_checker.alias_type_in_runtime_context(
-                    sym.node, sym.node.no_args, sym.node
-                )
+                return self.expr_checker.alias_type_in_runtime_context(sym.node, ctx=sym.node)
         # TODO: handle more node kinds here.
         return None
 

mypy/checkexpr.py

@@ -341,7 +341,7 @@ def analyze_ref_expr(self, e: RefExpr, lvalue: bool = False) -> Type:
             # Note that we suppress bogus errors for alias redefinitions,
             # they are already reported in semanal.py.
             result = self.alias_type_in_runtime_context(
-                node, node.no_args, e, alias_definition=e.is_alias_rvalue or lvalue
+                node, ctx=e, alias_definition=e.is_alias_rvalue or lvalue
             )
         elif isinstance(node, (TypeVarExpr, ParamSpecExpr)):
             result = self.object_type()
@@ -3805,12 +3805,10 @@ def visit_type_alias_expr(self, alias: TypeAliasExpr) -> Type:
         both `reveal_type` instances will reveal the same type `def (...) -> builtins.list[Any]`.
         Note that type variables are implicitly substituted with `Any`.
         """
-        return self.alias_type_in_runtime_context(
-            alias.node, alias.no_args, alias, alias_definition=True
-        )
+        return self.alias_type_in_runtime_context(alias.node, ctx=alias, alias_definition=True)
 
     def alias_type_in_runtime_context(
-        self, alias: TypeAlias, no_args: bool, ctx: Context, *, alias_definition: bool = False
+        self, alias: TypeAlias, *, ctx: Context, alias_definition: bool = False
     ) -> Type:
         """Get type of a type alias (could be generic) in a runtime expression.
 
@@ -3842,7 +3840,7 @@ class LongName(Generic[T]): ...
             # Normally we get a callable type (or overloaded) with .is_type_obj() true
             # representing the class's constructor
             tp = type_object_type(item.type, self.named_type)
-            if no_args:
+            if alias.no_args:
                 return tp
             return self.apply_type_arguments_to_callable(tp, item.args, ctx)
         elif (
@@ -3860,6 +3858,7 @@ class LongName(Generic[T]): ...
             if alias_definition:
                 return AnyType(TypeOfAny.special_form)
             # This type is invalid in most runtime contexts, give it an 'object' type.
+            # TODO: Use typing._SpecialForm instead?
             return self.named_type("builtins.object")
 
     def apply_type_arguments_to_callable(

mypy/checkmember.py

@@ -31,7 +31,6 @@
     is_final_node,
 )
 from mypy.plugin import AttributeContext
-from mypy.typeanal import set_any_tvars
 from mypy.typeops import (
     bind_self,
     class_callable,
@@ -458,14 +457,16 @@ def analyze_member_var_access(
         v = Var(name, type=type_object_type(vv, mx.named_type))
         v.info = info
 
-    if isinstance(vv, TypeAlias) and isinstance(get_proper_type(vv.target), Instance):
+    if isinstance(vv, TypeAlias):
         # Similar to the above TypeInfo case, we allow using
         # qualified type aliases in runtime context if it refers to an
         # instance type. For example:
         #     class C:
         #         A = List[int]
         #     x = C.A() <- this is OK
-        typ = instance_alias_type(vv, mx.named_type)
+        typ = mx.chk.expr_checker.alias_type_in_runtime_context(
+            vv, ctx=mx.context, alias_definition=mx.is_lvalue
+        )
         v = Var(name, type=typ)
         v.info = info
 
@@ -657,21 +658,6 @@ def analyze_descriptor_access(descriptor_type: Type, mx: MemberContext) -> Type:
     return inferred_dunder_get_type.ret_type
 
 
-def instance_alias_type(alias: TypeAlias, named_type: Callable[[str], Instance]) -> Type:
-    """Type of a type alias node targeting an instance, when appears in runtime context.
-
-    As usual, we first erase any unbound type variables to Any.
-    """
-    target: Type = get_proper_type(alias.target)
-    assert isinstance(
-        get_proper_type(target), Instance
-    ), "Must be called only with aliases to classes"
-    target = get_proper_type(set_any_tvars(alias, alias.line, alias.column))
-    assert isinstance(target, Instance)
-    tp = type_object_type(target.type, named_type)
-    return expand_type_by_instance(tp, target)
-
-
 def is_instance_var(var: Var, info: TypeInfo) -> bool:
     """Return if var is an instance variable according to PEP 526."""
     return (
@@ -980,10 +966,10 @@ def analyze_class_attribute_access(
         # Reference to a module object.
         return mx.named_type("types.ModuleType")
 
-    if isinstance(node.node, TypeAlias) and isinstance(
-        get_proper_type(node.node.target), Instance
-    ):
-        return instance_alias_type(node.node, mx.named_type)
+    if isinstance(node.node, TypeAlias):
+        return mx.chk.expr_checker.alias_type_in_runtime_context(
+            node.node, ctx=mx.context, alias_definition=mx.is_lvalue
+        )
 
     if is_decorated:
         assert isinstance(node.node, Decorator)

test-data/unit/check-dataclasses.test

@@ -632,8 +632,8 @@ class Two:
     S: TypeAlias = Callable[[int], str]  # E: Type aliases inside dataclass definitions are not supported at runtime
 
 c = Two()
-x = c.S  # E: Member "S" is not assignable
-reveal_type(x)  # N: Revealed type is "Any"
+x = c.S
+reveal_type(x)  # N: Revealed type is "builtins.object"
 [builtins fixtures/dataclasses.pyi]
 
 [case testDataclassOrdering]

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

@@ -796,3 +796,154 @@ S = TypeVar("S")
 class C(Generic[S], List[Defer]): ...
 class Defer: ...
 [builtins fixtures/list.pyi]
+
+[case testClassLevelTypeAliasesInUnusualContexts]
+from typing import Union
+from typing_extensions import TypeAlias
+
+class Foo: pass
+
+NormalImplicit = Foo
+NormalExplicit: TypeAlias = Foo
+SpecialImplicit = Union[int, str]
+SpecialExplicit: TypeAlias = Union[int, str]
+
+class Parent:
+    NormalImplicit = Foo
+    NormalExplicit: TypeAlias = Foo
+    SpecialImplicit = Union[int, str]
+    SpecialExplicit: TypeAlias = Union[int, str]
+
+class Child(Parent): pass
+
+p = Parent()
+c = Child()
+
+# Use type aliases in a runtime context
+
+reveal_type(NormalImplicit)   # N: Revealed type is "def () -> __main__.Foo"
+reveal_type(NormalExplicit)   # N: Revealed type is "def () -> __main__.Foo"
+reveal_type(SpecialImplicit)  # N: Revealed type is "builtins.object"
+reveal_type(SpecialExplicit)  # N: Revealed type is "builtins.object"
+
+reveal_type(Parent.NormalImplicit)   # N: Revealed type is "def () -> __main__.Foo"
+reveal_type(Parent.NormalExplicit)   # N: Revealed type is "def () -> __main__.Foo"
+reveal_type(Parent.SpecialImplicit)  # N: Revealed type is "builtins.object"
+reveal_type(Parent.SpecialExplicit)  # N: Revealed type is "builtins.object"
+
+reveal_type(Child.NormalImplicit)   # N: Revealed type is "def () -> __main__.Foo"
+reveal_type(Child.NormalExplicit)   # N: Revealed type is "def () -> __main__.Foo"
+reveal_type(Child.SpecialImplicit)  # N: Revealed type is "builtins.object"
+reveal_type(Child.SpecialExplicit)  # N: Revealed type is "builtins.object"
+
+reveal_type(p.NormalImplicit)   # N: Revealed type is "def () -> __main__.Foo"
+reveal_type(p.NormalExplicit)   # N: Revealed type is "def () -> __main__.Foo"
+reveal_type(p.SpecialImplicit)  # N: Revealed type is "builtins.object"
+reveal_type(p.SpecialExplicit)  # N: Revealed type is "builtins.object"
+
+reveal_type(c.NormalImplicit)   # N: Revealed type is "def () -> __main__.Foo"
+reveal_type(p.NormalExplicit)   # N: Revealed type is "def () -> __main__.Foo"
+reveal_type(c.SpecialImplicit)  # N: Revealed type is "builtins.object"
+reveal_type(c.SpecialExplicit)  # N: Revealed type is "builtins.object"
+
+# Use type aliases in a type alias context in a plausible way
+
+def plausible_top_1() -> NormalImplicit: pass
+def plausible_top_2() -> NormalExplicit: pass
+def plausible_top_3() -> SpecialImplicit: pass
+def plausible_top_4() -> SpecialExplicit: pass
+reveal_type(plausible_top_1)  # N: Revealed type is "def () -> __main__.Foo"
+reveal_type(plausible_top_2)  # N: Revealed type is "def () -> __main__.Foo"
+reveal_type(plausible_top_3)  # N: Revealed type is "def () -> Union[builtins.int, builtins.str]"
+reveal_type(plausible_top_4)  # N: Revealed type is "def () -> Union[builtins.int, builtins.str]"
+
+def plausible_parent_1() -> Parent.NormalImplicit: pass  # E: Variable "__main__.Parent.NormalImplicit" is not valid as a type \
+                                                         # N: See https://mypy.readthedocs.io/en/stable/common_issues.html#variables-vs-type-aliases
+def plausible_parent_2() -> Parent.NormalExplicit: pass
+def plausible_parent_3() -> Parent.SpecialImplicit: pass
+def plausible_parent_4() -> Parent.SpecialExplicit: pass
+reveal_type(plausible_parent_1)  # N: Revealed type is "def () -> Parent.NormalImplicit?"
+reveal_type(plausible_parent_2)  # N: Revealed type is "def () -> __main__.Foo"
+reveal_type(plausible_parent_3)  # N: Revealed type is "def () -> Union[builtins.int, builtins.str]"
+reveal_type(plausible_parent_4)  # N: Revealed type is "def () -> Union[builtins.int, builtins.str]"
+
+def plausible_child_1() -> Child.NormalImplicit: pass  # E: Variable "__main__.Parent.NormalImplicit" is not valid as a type \
+                                                       # N: See https://mypy.readthedocs.io/en/stable/common_issues.html#variables-vs-type-aliases
+def plausible_child_2() -> Child.NormalExplicit: pass
+def plausible_child_3() -> Child.SpecialImplicit: pass
+def plausible_child_4() -> Child.SpecialExplicit: pass
+reveal_type(plausible_child_1)  # N: Revealed type is "def () -> Child.NormalImplicit?"
+reveal_type(plausible_child_2)  # N: Revealed type is "def () -> __main__.Foo"
+reveal_type(plausible_child_3)  # N: Revealed type is "def () -> Union[builtins.int, builtins.str]"
+reveal_type(plausible_child_4)  # N: Revealed type is "def () -> Union[builtins.int, builtins.str]"
+
+# Use type aliases in a type alias context in an implausible way
+
+def weird_parent_1() -> p.NormalImplicit: pass   # E: Name "p.NormalImplicit" is not defined
+def weird_parent_2() -> p.NormalExplicit: pass   # E: Name "p.NormalExplicit" is not defined
+def weird_parent_3() -> p.SpecialImplicit: pass  # E: Name "p.SpecialImplicit" is not defined
+def weird_parent_4() -> p.SpecialExplicit: pass  # E: Name "p.SpecialExplicit" is not defined
+reveal_type(weird_parent_1)  # N: Revealed type is "def () -> Any"
+reveal_type(weird_parent_2)  # N: Revealed type is "def () -> Any"
+reveal_type(weird_parent_3)  # N: Revealed type is "def () -> Any"
+reveal_type(weird_parent_4)  # N: Revealed type is "def () -> Any"
+
+def weird_child_1() -> c.NormalImplicit: pass   # E: Name "c.NormalImplicit" is not defined
+def weird_child_2() -> c.NormalExplicit: pass   # E: Name "c.NormalExplicit" is not defined
+def weird_child_3() -> c.SpecialImplicit: pass  # E: Name "c.SpecialImplicit" is not defined
+def weird_child_4() -> c.SpecialExplicit: pass  # E: Name "c.SpecialExplicit" is not defined
+reveal_type(weird_child_1)  # N: Revealed type is "def () -> Any"
+reveal_type(weird_child_2)  # N: Revealed type is "def () -> Any"
+reveal_type(weird_child_3)  # N: Revealed type is "def () -> Any"
+reveal_type(weird_child_4)  # N: Revealed type is "def () -> Any"
+[builtins fixtures/tuple.pyi]
+
+[case testMalformedTypeAliasRuntimeReassignments]
+from typing import Union
+from typing_extensions import TypeAlias
+
+class Foo: pass
+
+NormalImplicit = Foo
+NormalExplicit: TypeAlias = Foo
+SpecialImplicit = Union[int, str]
+SpecialExplicit: TypeAlias = Union[int, str]
+
+class Parent:
+    NormalImplicit = Foo
+    NormalExplicit: TypeAlias = Foo
+    SpecialImplicit = Union[int, str]
+    SpecialExplicit: TypeAlias = Union[int, str]
+
+class Child(Parent): pass
+
+p = Parent()
+c = Child()
+
+NormalImplicit = 4   # E: Cannot assign multiple types to name "NormalImplicit" without an explicit "Type[...]" annotation \
+                     # E: Incompatible types in assignment (expression has type "int", variable has type "Type[Foo]")
+NormalExplicit = 4   # E: Cannot assign multiple types to name "NormalExplicit" without an explicit "Type[...]" annotation \
+                     # E: Incompatible types in assignment (expression has type "int", variable has type "Type[Foo]")
+SpecialImplicit = 4  # E: Cannot assign multiple types to name "SpecialImplicit" without an explicit "Type[...]" annotation
+SpecialExplicit = 4  # E: Cannot assign multiple types to name "SpecialExplicit" without an explicit "Type[...]" annotation
+
+Parent.NormalImplicit = 4  # E: Incompatible types in assignment (expression has type "int", variable has type "Type[Foo]")
+Parent.NormalExplicit = 4  # E: Incompatible types in assignment (expression has type "int", variable has type "Type[Foo]")
+Parent.SpecialImplicit = 4
+Parent.SpecialExplicit = 4
+
+Child.NormalImplicit = 4  # E: Incompatible types in assignment (expression has type "int", variable has type "Type[Foo]")
+Child.NormalExplicit = 4  # E: Incompatible types in assignment (expression has type "int", variable has type "Type[Foo]")
+Child.SpecialImplicit = 4
+Child.SpecialExplicit = 4
+
+p.NormalImplicit = 4  # E: Incompatible types in assignment (expression has type "int", variable has type "Type[Foo]")
+p.NormalExplicit = 4  # E: Incompatible types in assignment (expression has type "int", variable has type "Type[Foo]")
+p.SpecialImplicit = 4
+p.SpecialExplicit = 4
+
+c.NormalImplicit = 4  # E: Incompatible types in assignment (expression has type "int", variable has type "Type[Foo]")
+c.NormalExplicit = 4  # E: Incompatible types in assignment (expression has type "int", variable has type "Type[Foo]")
+c.SpecialImplicit = 4
+c.SpecialExplicit = 4
+[builtins fixtures/tuple.pyi]