Strip PostgreSQL type cast specifiers when comparing computed defaults

alembic/autogenerate/compare/server_defaults.py

@@ -55,7 +55,10 @@ def _normalize_computed_default(sqltext: str) -> str:
 
     """
 
-    return re.sub(r"[ \(\)'\"`\[\]\t\r\n]", "", sqltext).lower()
+    normalized_sqltext = re.sub(r"[ \(\)'\"`\[\]\t\r\n]", "", sqltext).lower()
+    # strip postgresql type cast specifiers, e.g. ``::regconfig``, ``::text``,
+    # which can appear inconsistently and cause false-positive warnings.
+    return re.sub(r"::\w+", "", normalized_sqltext)
 
 
 def _compare_computed_default(

docs/build/unreleased/1462.rst

@@ -0,0 +1,10 @@
+.. change::
+    :tags: bug, autogenerate, postgresql
+    :tickets: 1462
+
+    Fixed false-positive "Computed default cannot be modified" warning emitted
+    during autogenerate when comparing PostgreSQL ``Computed`` columns whose
+    reflected SQL expression includes type cast specifiers such as
+    ``::regconfig`` or ``::text``.  These cast specifiers are now stripped
+    before comparison so that semantically equivalent expressions no longer
+    trigger the warning.

tests/test_postgresql.py

@@ -72,6 +72,9 @@
     from alembic.autogenerate.compare.server_defaults import (
         _dialect_impl_compare_server_default as _compare_server_default,
     )
+    from alembic.autogenerate.compare.server_defaults import (
+        _normalize_computed_default,
+    )
 
 
 class PostgresqlOpTest(TestBase):
@@ -395,6 +398,20 @@ def test_alter_column_computed_not_supported(self, sd, esd):
             existing_server_default=esd(),
         )
 
+    def test_normalize_computed_default_strips_type_casts(self):
+        # type cast specifiers such as ``::regconfig`` only appear on the
+        # reflected (connection) side and would otherwise cause a
+        # false-positive "cannot be modified" warning.  issue #1462
+        eq_(
+            _normalize_computed_default(
+                "setweight(to_tsvector('english', title), 'a')"
+            ),
+            _normalize_computed_default(
+                "setweight(to_tsvector('english'::regconfig, "
+                "title::text), 'a'::\"char\")"
+            ),
+        )
+
     @combinations(
         ({}, None),
         (dict(always=True), None),