From b0978c50ed8509761fa71ece054bc9aae64177d7 Mon Sep 17 00:00:00 2001
From: Andrew Hannigan <andrewhannigan@Andrews-MacBook-Pro.local>
Date: Wed, 18 Nov 2020 09:55:01 -0600
Subject: [PATCH] Implement TypeEngine.as_generic() and tests

Fixes: #5659
---
 lib/sqlalchemy/sql/type_api.py |  7 ++++++-
 test/sql/test_types.py         | 20 ++++++++++++++++++++
 2 files changed, 26 insertions(+), 1 deletion(-)

diff --git a/lib/sqlalchemy/sql/type_api.py b/lib/sqlalchemy/sql/type_api.py
index bca6e9020e..e4d66c54f7 100644
--- a/lib/sqlalchemy/sql/type_api.py
+++ b/lib/sqlalchemy/sql/type_api.py
@@ -470,7 +470,7 @@ class TypeEngine(Traversible):
                     "sqlalchemy.sql.sqltypes",
                     "sqlalchemy.sql.type_api",
                 )
-                and t._is_generic_type()
+                and hasattr(t, '_is_generic_type') and t._is_generic_type()
             ):
                 if t in (TypeEngine, UserDefinedType):
                     return NULLTYPE.__class__
@@ -478,6 +478,11 @@ class TypeEngine(Traversible):
         else:
             return self.__class__
 
+    def as_generic(self):
+        """Return an instance of the generic type corresponding to this type"""
+
+        return util.constructor_copy(self, self._generic_type_affinity())
+
     def dialect_impl(self, dialect):
         """Return a dialect-specific implementation for this
         :class:`.TypeEngine`.
diff --git a/test/sql/test_types.py b/test/sql/test_types.py
index fd1783e098..570383dfed 100644
--- a/test/sql/test_types.py
+++ b/test/sql/test_types.py
@@ -383,6 +383,26 @@ class TypeAffinityTest(fixtures.TestBase):
         assert t1.dialect_impl(d)._type_affinity is postgresql.UUID
 
 
+class AsGenericTest(fixtures.TestBase):
+    @testing.combinations(
+        (String(), String()),
+        (VARCHAR(length=100), String(length=100)),
+        (NVARCHAR(length=100), Unicode(length=100)),
+        (DATE(), Date()),
+    )
+    def test_as_generic(self, t1, t2):
+        assert repr(t1.as_generic()) == repr(t2)
+
+    @testing.combinations(*[(t,) for t in _all_types(omit_special_types=True)])
+    def test_as_generic_all_types(self, type_):
+        if issubclass(type_, ARRAY):
+            t1 = type_(String)
+        else:
+            t1 = type_()
+
+        t1.as_generic()
+
+
 class PickleTypesTest(fixtures.TestBase):
     @testing.combinations(
         ("Boo", Boolean()),
-- 
2.47.3