disallow same-named columns, unchecked replacement in Table

Fixed issue where table reflection using :paramref:`.Table.extend_existing`
would fail to deduplicate a same-named column if the existing
:class:`.Table` used a separate key. The
:paramref:`.Table.autoload_replace` parameter would allow the column to be
skipped but under no circumstances should a :class:`.Table` ever have the
same-named column twice.

Additionally, changed deprecation warnings to exceptions
as were implemented in I1d58c8ebe081079cb669e7ead60886ffc1b1a7f5 .

Fixes: #8925
Change-Id: I83d0f8658177a7ffbb06e01dbca91377d1a98d49

2 files changed, 24 insertions, 6 deletions

diff --git a/lib/sqlalchemy/testing/__init__.py b/lib/sqlalchemy/testing/__init__.py
index 3a028f002..993fc4954 100644
--- a/lib/sqlalchemy/testing/__init__.py
+++ b/lib/sqlalchemy/testing/__init__.py
@@ -54,6 +54,7 @@ from .config import db
 from .config import fixture
 from .config import requirements as requires
 from .config import skip_test
+from .config import Variation
 from .config import variation
 from .exclusions import _is_excluded
 from .exclusions import _server_version
diff --git a/lib/sqlalchemy/testing/config.py b/lib/sqlalchemy/testing/config.py
index a75c36776..957876579 100644
--- a/lib/sqlalchemy/testing/config.py
+++ b/lib/sqlalchemy/testing/config.py
@@ -14,11 +14,13 @@ import typing
 from typing import Any
 from typing import Callable
 from typing import Iterable
+from typing import NoReturn
 from typing import Optional
 from typing import Tuple
 from typing import TypeVar
 from typing import Union
 
+from .util import fail
 from .. import util
 
 requirements = None
@@ -128,21 +130,36 @@ def combinations_list(
     return combinations(*arg_iterable, **kw)
 
 
-class _variation_base:
-    __slots__ = ("name", "argname")
+class Variation:
+    __slots__ = ("_name", "_argname")
 
     def __init__(self, case, argname, case_names):
-        self.name = case
-        self.argname = argname
+        self._name = case
+        self._argname = argname
         for casename in case_names:
             setattr(self, casename, casename == case)
 
+    if typing.TYPE_CHECKING:
+
+        def __getattr__(self, key: str) -> bool:
+            ...
+
+    @property
+    def name(self):
+        return self._name
+
     def __bool__(self):
-        return self.name == self.argname
+        return self._name == self._argname
 
     def __nonzero__(self):
         return not self.__bool__()
 
+    def __str__(self):
+        return f"{self._argname}={self._name!r}"
+
+    def fail(self) -> NoReturn:
+        fail(f"Unknown {self}")
+
 
 def variation(argname, cases):
     """a helper around testing.combinations that provides a single namespace
@@ -193,7 +210,7 @@ def variation(argname, cases):
 
     typ = type(
         argname,
-        (_variation_base,),
+        (Variation,),
         {
             "__slots__": tuple(case_names),
         },