pep484 ORM / SQL result support

after some experimentation it seems mypy is more amenable
to the generic types being fully integrated rather than
having separate spin-off types.   so key structures
like Result, Row, Select become generic.  For DML
Insert, Update, Delete, these are spun into type-specific
subclasses ReturningInsert, ReturningUpdate, ReturningDelete,
which is fine since the "row-ness" of these constructs
doesn't happen until returning() is called in any case.

a Tuple based model is then integrated so that these
objects can carry along information about their return
types.  Overloads at the .execute() level carry through
the Tuple from the invoked object to the result.

To suit the issue of AliasedClass generating attributes
that are dynamic, experimented with a custom subclass
AsAliased, but then just settled on having aliased()
lie to the type checker and return `Type[_O]`, essentially.
will need some type-related accessors for with_polymorphic()
also.

Additionally, identified an issue in Update when used
"mysql style" against a join(), it basically doesn't work
if asked to UPDATE two tables on the same column name.
added an error message to the specific condition where
it happens with a very non-specific error message that we
hit a thing we can't do right now, suggest multi-table
update as a possible cause.

Change-Id: I5eff7eefe1d6166ee74160b2785c5e6a81fa8b95

1 files changed, 6 insertions, 7 deletions

diff --git a/lib/sqlalchemy/orm/mapper.py b/lib/sqlalchemy/orm/mapper.py
index b37c080ea..083035093 100644
--- a/lib/sqlalchemy/orm/mapper.py
+++ b/lib/sqlalchemy/orm/mapper.py
@@ -96,7 +96,6 @@ if TYPE_CHECKING:
     from .descriptor_props import Synonym
     from .events import MapperEvents
     from .instrumentation import ClassManager
-    from .path_registry import AbstractEntityRegistry
     from .path_registry import CachingEntityRegistry
     from .properties import ColumnProperty
     from .relationships import Relationship
@@ -108,10 +107,10 @@ if TYPE_CHECKING:
     from ..sql.base import ReadOnlyColumnCollection
     from ..sql.elements import ColumnClause
     from ..sql.elements import ColumnElement
+    from ..sql.elements import KeyedColumnElement
     from ..sql.schema import Column
     from ..sql.schema import Table
     from ..sql.selectable import FromClause
-    from ..sql.selectable import TableClause
     from ..sql.util import ColumnAdapter
     from ..util import OrderedSet
 
@@ -161,7 +160,7 @@ _CONFIGURE_MUTEX = threading.RLock()
 @log.class_logger
 class Mapper(
     ORMFromClauseRole,
-    ORMEntityColumnsClauseRole,
+    ORMEntityColumnsClauseRole[_O],
     MemoizedHasCacheKey,
     InspectionAttr,
     log.Identified,
@@ -1006,7 +1005,7 @@ class Mapper(
 
     """
 
-    polymorphic_on: Optional[ColumnElement[Any]]
+    polymorphic_on: Optional[KeyedColumnElement[Any]]
     """The :class:`_schema.Column` or SQL expression specified as the
     ``polymorphic_on`` argument
     for this :class:`_orm.Mapper`, within an inheritance scenario.
@@ -1699,10 +1698,10 @@ class Mapper(
                     instrument = True
                 key = getattr(col, "key", None)
                 if key:
-                    if self._should_exclude(col.key, col.key, False, col):
+                    if self._should_exclude(key, key, False, col):
                         raise sa_exc.InvalidRequestError(
                             "Cannot exclude or override the "
-                            "discriminator column %r" % col.key
+                            "discriminator column %r" % key
                         )
                 else:
                     self.polymorphic_on = col = col.label("_sa_polymorphic_on")
@@ -2948,7 +2947,7 @@ class Mapper(
 
     def identity_key_from_row(
         self,
-        row: Optional[Union[Row, RowMapping]],
+        row: Optional[Union[Row[Any], RowMapping]],
         identity_token: Optional[Any] = None,
         adapter: Optional[ColumnAdapter] = None,
     ) -> _IdentityKeyType[_O]: