Add concept of "implicit boolean", treat as native

Fixed issue that is closely related to :ticket:`3639` where an expression
rendered in a boolean context on a non-native boolean backend would
be compared to 1/0 even though it is already an implcitly boolean
expression, when :meth:`.ColumnElement.self_group` were used.  While this
does not affect the user-friendly backends (MySQL, SQLite) it was not
handled by Oracle (and possibly SQL Server).   Whether or not the
expression is implicitly boolean on any database is now determined
up front as an additional check to not generate the integer comparison
within the compliation of the statement.

Fixes: #4320
Change-Id: Iae0a65e5c01bd576e64733c3651e1e1a1a1b240c

1 files changed, 4 insertions, 2 deletions

diff --git a/lib/sqlalchemy/sql/compiler.py b/lib/sqlalchemy/sql/compiler.py
index 5fbc7d87f..e45db428a 100644
--- a/lib/sqlalchemy/sql/compiler.py
+++ b/lib/sqlalchemy/sql/compiler.py
@@ -1019,13 +1019,15 @@ class SQLCompiler(Compiled):
                 "Unary expression has no operator or modifier")
 
     def visit_istrue_unary_operator(self, element, operator, **kw):
-        if self.dialect.supports_native_boolean:
+        if element._is_implicitly_boolean or \
+                self.dialect.supports_native_boolean:
             return self.process(element.element, **kw)
         else:
             return "%s = 1" % self.process(element.element, **kw)
 
     def visit_isfalse_unary_operator(self, element, operator, **kw):
-        if self.dialect.supports_native_boolean:
+        if element._is_implicitly_boolean or \
+                self.dialect.supports_native_boolean:
             return "NOT %s" % self.process(element.element, **kw)
         else:
             return "%s = 0" % self.process(element.element, **kw)