add testTypeVarBoundToNewUnionAttributeAccess

Author: DetachHead

mypy/test/testcheck.py

@@ -104,6 +104,8 @@
     typecheck_files.append('check-python38.test')
 if sys.version_info >= (3, 9):
     typecheck_files.append('check-python39.test')
+if sys.version_info >= (3, 10):
+    typecheck_files.append('check-python310.test')
 
 # Special tests for platforms with case-insensitive filesystems.
 if sys.platform in ('darwin', 'win32'):

test-data/unit/check-generic-subtyping.test

@@ -818,7 +818,7 @@ class Y(Generic[T]):
         return U()  # E: Incompatible return value type (got "U", expected "T")
 
 
-[case testTypeVarBoundToUnionAttributeAccess]
+[case testTypeVarBoundToOldUnionAttributeAccess]
 from typing import Union, TypeVar
 
 class U:

test-data/unit/check-python310.test

@@ -0,0 +1,24 @@
+[case testTypeVarBoundToNewUnionAttributeAccess]
+from typing import TypeVar
+
+class U:
+    a: float
+class V:
+    b: float
+class W:
+    c: float
+
+T = TypeVar("T", bound=U | V | W)
+
+def f(x: T) -> None:
+    x.a  # E
+    x.b = 1.0  # E
+    del x.c  # E
+
+[out]
+main:13: error: Item "V" of the upper bound "Union[U, V, W]" of type variable "T" has no attribute "a"
+main:13: error: Item "W" of the upper bound "Union[U, V, W]" of type variable "T" has no attribute "a"
+main:14: error: Item "U" of the upper bound "Union[U, V, W]" of type variable "T" has no attribute "b"
+main:14: error: Item "W" of the upper bound "Union[U, V, W]" of type variable "T" has no attribute "b"
+main:15: error: Item "U" of the upper bound "Union[U, V, W]" of type variable "T" has no attribute "c"
+main:15: error: Item "V" of the upper bound "Union[U, V, W]" of type variable "T" has no attribute "c"