Report attribute access errors for TypeVar bound to Union (#11140)

Author: tyralla

mypy/messages.py

@@ -345,6 +345,16 @@ def has_no_attr(self,
                 self.fail('Item {} of {} has no attribute "{}"{}'.format(
                     typ_format, orig_type_format, member, extra), context,
                     code=codes.UNION_ATTR)
+            elif isinstance(original_type, TypeVarType):
+                bound = get_proper_type(original_type.upper_bound)
+                if isinstance(bound, UnionType):
+                    typ_fmt, bound_fmt = format_type_distinctly(typ, bound)
+                    original_type_fmt = format_type(original_type)
+                    self.fail(
+                        'Item {} of the upper bound {} of type variable {} has no '
+                        'attribute "{}"{}'.format(
+                            typ_fmt, bound_fmt, original_type_fmt, member, extra),
+                        context, code=codes.UNION_ATTR)
         return AnyType(TypeOfAny.from_error)
 
     def unsupported_operand_types(self,

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

@@ -817,4 +817,28 @@ class Y(Generic[T]):
     def f(self) -> T:
         return U()  # E: Incompatible return value type (got "U", expected "T")
 
+
+[case testTypeVarBoundToUnionAttributeAccess]
+from typing import Union, TypeVar
+
+class U:
+    a: float
+class V:
+    b: float
+class W:
+    c: float
+
+T = TypeVar("T", bound=Union[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"