Support union type syntax in type applications, aliases and casts

These are only available in Python 3.10 mode.

Work on #9880.

mypy/exprtotype.py

@@ -4,13 +4,13 @@
 
 from mypy.nodes import (
     Expression, NameExpr, MemberExpr, IndexExpr, RefExpr, TupleExpr, IntExpr, FloatExpr, UnaryExpr,
-    ComplexExpr, ListExpr, StrExpr, BytesExpr, UnicodeExpr, EllipsisExpr, CallExpr,
+    ComplexExpr, ListExpr, StrExpr, BytesExpr, UnicodeExpr, EllipsisExpr, CallExpr, OpExpr,
     get_member_expr_fullname
 )
 from mypy.fastparse import parse_type_string
 from mypy.types import (
     Type, UnboundType, TypeList, EllipsisType, AnyType, CallableArgument, TypeOfAny,
-    RawExpressionType, ProperType
+    RawExpressionType, ProperType, UnionType
 )
 
 
@@ -150,5 +150,8 @@ def expr_to_unanalyzed_type(expr: Expression, _parent: Optional[Expression] = No
         return RawExpressionType(None, 'builtins.complex', line=expr.line, column=expr.column)
     elif isinstance(expr, EllipsisExpr):
         return EllipsisType(expr.line)
+    elif isinstance(expr, OpExpr) and expr.op == '|':
+        return UnionType([expr_to_unanalyzed_type(expr.left),
+                          expr_to_unanalyzed_type(expr.right)])
     else:
         raise TypeTranslationError()

mypy/semanal.py

@@ -2101,7 +2101,7 @@ def should_wait_rhs(self, rv: Expression) -> bool:
             return self.should_wait_rhs(rv.callee)
         return False
 
-    def can_be_type_alias(self, rv: Expression) -> bool:
+    def can_be_type_alias(self, rv: Expression, allow_none: bool = False) -> bool:
         """Is this a valid r.h.s. for an alias definition?
 
         Note: this function should be only called for expressions where self.should_wait_rhs()
@@ -2113,6 +2113,13 @@ def can_be_type_alias(self, rv: Expression) -> bool:
             return True
         if self.is_none_alias(rv):
             return True
+        if allow_none and isinstance(rv, NameExpr) and rv.fullname == 'builtins.None':
+            return True
+        if (isinstance(rv, OpExpr)
+                and rv.op == '|'
+                and self.can_be_type_alias(rv.left, allow_none=True)
+                and self.can_be_type_alias(rv.right, allow_none=True)):
+            return True
         return False
 
     def is_type_ref(self, rv: Expression, bare: bool = False) -> bool:

test-data/unit/check-classes.test

@@ -3154,7 +3154,7 @@ def foo(arg: Type[Any]):
 from typing import Type, Any
 def foo(arg: Type[Any]):
     reveal_type(arg.__str__)  # N: Revealed type is "def () -> builtins.str"
-    reveal_type(arg.mro())  # N: Revealed type is "builtins.list[builtins.type]"
+    reveal_type(arg.mro())  # N: Revealed type is "builtins.list[builtins.type[Any]]"
 [builtins fixtures/type.pyi]
 [out]
 

test-data/unit/check-union-or-syntax.test

@@ -96,10 +96,22 @@ reveal_type(y)  # N: Revealed type is "Union[builtins.int, builtins.str]"
 
 [case testUnionOrSyntaxWithTypeAliasWorking]
 # flags: --python-version 3.10
-from typing import Union
-T = Union[int, str]
+T = int | str
 x: T
 reveal_type(x)  # N: Revealed type is "Union[builtins.int, builtins.str]"
+S = list[int] | str | None
+y: S
+reveal_type(y)  # N: Revealed type is "Union[builtins.list[builtins.int], builtins.str, None]"
+U = str | None
+z: U
+reveal_type(z)  # N: Revealed type is "Union[builtins.str, None]"
+
+def f(): pass
+
+X = int | str | f()
+b: X  # E: Variable "__main__.X" is not valid as a type \
+    # N: See https://mypy.readthedocs.io/en/stable/common_issues.html#variables-vs-type-aliases
+[builtins fixtures/type.pyi]
 
 
 [case testUnionOrSyntaxWithTypeAliasNotAllowed]
@@ -131,3 +143,17 @@ x: int | None  # E: X | Y syntax for unions requires Python 3.10
 from lib import x
 [file lib.pyi]
 x: int | None
+
+
+[case testUnionOrSyntaxInMiscRuntimeContexts]
+# flags: --python-version 3.10
+from typing import cast
+
+class C(list[int | None]):
+    pass
+
+def f() -> object: pass
+
+reveal_type(cast(str | None, f()))  # N: Revealed type is "Union[builtins.str, None]"
+reveal_type(list[str | None]())  # N: Revealed type is "builtins.list[Union[builtins.str, None]]"
+[builtins fixtures/type.pyi]

test-data/unit/fixtures/type.pyi

@@ -1,6 +1,6 @@
 # builtins stub used in type-related test cases.
 
-from typing import Generic, TypeVar, List
+from typing import Generic, TypeVar, List, Union
 
 T = TypeVar('T')
 
@@ -10,8 +10,9 @@ class object:
 
 class list(Generic[T]): pass
 
-class type:
+class type(Generic[T]):
     __name__: str
+    def __or__(self, other: Union[type, None]) -> type: pass
     def mro(self) -> List['type']: pass
 
 class tuple(Generic[T]): pass