Support DEFAULT VALUES and VALUES(DEFAULT) individually

Fixed regression where the introduction of the INSERT syntax "INSERT...
VALUES (DEFAULT)" was not supported on some backends that do however
support "INSERT..DEFAULT VALUES", including SQLite. The two syntaxes are
now each individually supported or non-supported for each dialect, for
example MySQL supports "VALUES (DEFAULT)" but not "DEFAULT VALUES".
Support for Oracle is still not enabled as there are unresolved issues
in using RETURNING at the same time.

Fixes: #6254
Change-Id: I47959bc826e3d9d2396ccfa290eb084841b02e77

test/sql/test_defaults.py

@@ -2,7 +2,6 @@ import datetime
 import itertools
 
 import sqlalchemy as sa
-from sqlalchemy import Boolean
 from sqlalchemy import cast
 from sqlalchemy import DateTime
 from sqlalchemy import exc
@@ -1062,25 +1061,6 @@ class PKIncrementTest(fixtures.TablesTest):
         )
 
 
-class EmptyInsertTest(fixtures.TestBase):
-    __backend__ = True
-
-    @testing.fails_on("oracle", "FIXME: unknown")
-    def test_empty_insert(self, metadata, connection):
-        t1 = Table(
-            "t1",
-            metadata,
-            Column("is_true", Boolean, server_default=("1")),
-        )
-        metadata.create_all(connection)
-        connection.execute(t1.insert())
-        eq_(
-            1,
-            connection.scalar(select(func.count(text("*"))).select_from(t1)),
-        )
-        eq_(True, connection.scalar(t1.select()))
-
-
 class AutoIncrementTest(fixtures.TestBase):
 
     __backend__ = True
@@ -1088,7 +1068,11 @@ class AutoIncrementTest(fixtures.TestBase):
     @testing.requires.empty_inserts
     def test_autoincrement_single_col(self, metadata, connection):
         single = Table(
-            "single", self.metadata, Column("id", Integer, primary_key=True)
+            "single",
+            self.metadata,
+            Column(
+                "id", Integer, primary_key=True, test_needs_autoincrement=True
+            ),
         )
         self.metadata.create_all(connection)
 

test/sql/test_insert.py

@@ -951,7 +951,9 @@ class EmptyTest(_InsertTestBase, fixtures.TablesTest, AssertsCompiledSQL):
         table1 = self.tables.mytable
 
         dialect = default.DefaultDialect()
-        dialect.supports_empty_insert = dialect.supports_default_values = True
+        dialect.supports_empty_insert = False
+        dialect.supports_default_values = True
+        dialect.supports_default_metavalue = True
 
         stmt = table1.insert().values({})
         self.assert_compile(
@@ -961,6 +963,14 @@ class EmptyTest(_InsertTestBase, fixtures.TablesTest, AssertsCompiledSQL):
             for_executemany=True,
         )
 
+        dialect.supports_default_metavalue = False
+        self.assert_compile(
+            stmt,
+            "INSERT INTO mytable DEFAULT VALUES",
+            dialect=dialect,
+            for_executemany=True,
+        )
+
     def test_supports_empty_insert_false(self):
         table1 = self.tables.mytable