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, 5 insertions, 2 deletions

diff --git a/lib/sqlalchemy/dialects/postgresql/base.py b/lib/sqlalchemy/dialects/postgresql/base.py
index ceb624644..f18bec932 100644
--- a/lib/sqlalchemy/dialects/postgresql/base.py
+++ b/lib/sqlalchemy/dialects/postgresql/base.py
@@ -932,9 +932,11 @@ from ... import sql
 from ... import util
 from ...engine import default
 from ...engine import reflection
+from ...sql import coercions
 from ...sql import compiler
 from ...sql import elements
 from ...sql import expression
+from ...sql import roles
 from ...sql import sqltypes
 from ...sql import util as sql_util
 from ...types import BIGINT
@@ -1774,7 +1776,7 @@ class PGCompiler(compiler.SQLCompiler):
             col_key = c.key
             if col_key in set_parameters:
                 value = set_parameters.pop(col_key)
-                if elements._is_literal(value):
+                if coercions._is_literal(value):
                     value = elements.BindParameter(None, value, type_=c.type)
 
                 else:
@@ -1806,7 +1808,8 @@ class PGCompiler(compiler.SQLCompiler):
                     else self.process(k, use_schema=False)
                 )
                 value_text = self.process(
-                    elements._literal_as_binds(v), use_schema=False
+                    coercions.expect(roles.ExpressionElementRole, v),
+                    use_schema=False,
                 )
                 action_set_ops.append("%s = %s" % (key_text, value_text))