Implement new ClauseElement role and coercion system

A major refactoring of all the functions handle all detection of
Core argument types as well as perform coercions into a new class hierarchy
based on "roles", each of which identify a syntactical location within a
SQL statement.  In contrast to the ClauseElement hierarchy that identifies
"what" each object is syntactically, the SQLRole hierarchy identifies
the "where does it go" of each object syntactically.   From this we define
a consistent type checking and coercion system that establishes well
defined behviors.

This is a breakout of the patch that is reorganizing select()
constructs to no longer be in the FromClause hierarchy.

Also includes a rename of as_scalar() into scalar_subquery(); deprecates
automatic coercion to scalar_subquery().

Partially-fixes: #4617
Change-Id: I26f1e78898693c6b99ef7ea2f4e7dfd0e8e1a1bd

1 files changed, 3 insertions, 3 deletions

diff --git a/test/sql/test_metadata.py b/test/sql/test_metadata.py
index 3d60fb60e..3f9667609 100644
--- a/test/sql/test_metadata.py
+++ b/test/sql/test_metadata.py
@@ -31,7 +31,6 @@ from sqlalchemy import Unicode
 from sqlalchemy import UniqueConstraint
 from sqlalchemy import util
 from sqlalchemy.engine import default
-from sqlalchemy.sql import elements
 from sqlalchemy.sql import naming
 from sqlalchemy.testing import assert_raises
 from sqlalchemy.testing import assert_raises_message
@@ -3378,7 +3377,8 @@ class ConstraintTest(fixtures.TestBase):
 
         assert_raises_message(
             exc.ArgumentError,
-            r"Element Table\('t2', .* is not a string name or column element",
+            r"String column name or column object for DDL constraint "
+            r"expected, got .*SomeClass",
             Index,
             "foo",
             SomeClass(),
@@ -4609,7 +4609,7 @@ class NamingConventionTest(fixtures.TestBase, AssertsCompiledSQL):
         u1 = self._fixture(
             naming_convention={"ck": "ck_%(table_name)s_%(constraint_name)s"}
         )
-        ck = CheckConstraint(u1.c.data == "x", name=elements._defer_name(None))
+        ck = CheckConstraint(u1.c.data == "x", name=naming._defer_name(None))
 
         assert_raises_message(
             exc.InvalidRequestError,