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

4 files changed, 32 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)
diff --git a/lib/sqlalchemy/sql/elements.py b/lib/sqlalchemy/sql/elements.py
index 2e8c39f3b..2a6fa323c 100644
--- a/lib/sqlalchemy/sql/elements.py
+++ b/lib/sqlalchemy/sql/elements.py
@@ -641,6 +641,8 @@ class ColumnElement(operators.ColumnOperators, ClauseElement):
     """A flag that can be flipped to prevent a column from being resolvable
     by string label name."""
 
+    _is_implicitly_boolean = False
+
     _alt_names = ()
 
     def self_group(self, against=None):
@@ -1229,6 +1231,7 @@ class TextClause(Executable, ClauseElement):
     _execution_options = \
         Executable._execution_options.union(
             {'autocommit': PARSE_AUTOCOMMIT})
+    _is_implicitly_boolean = False
 
     @property
     def _select_iterable(self):
@@ -1813,6 +1816,7 @@ class ClauseList(ClauseElement):
             self.clauses = [
                 text_converter(clause)
                 for clause in clauses]
+        self._is_implicitly_boolean = operators.is_boolean(self.operator)
 
     def __iter__(self):
         return iter(self.clauses)
@@ -1915,6 +1919,7 @@ class BooleanClauseList(ClauseList, ColumnElement):
         self.operator = operator
         self.group_contents = True
         self.type = type_api.BOOLEANTYPE
+        self._is_implicitly_boolean = True
         return self
 
     @classmethod
@@ -2923,6 +2928,7 @@ class AsBoolean(UnaryExpression):
         self.negate = negate
         self.modifier = None
         self.wraps_column_expression = True
+        self._is_implicitly_boolean = element._is_implicitly_boolean
 
     def self_group(self, against=None):
         return self
@@ -2950,6 +2956,12 @@ class BinaryExpression(ColumnElement):
 
     __visit_name__ = 'binary'
 
+    _is_implicitly_boolean = True
+    """Indicates that any database will know this is a boolean expression
+    even if the database does not have an explicit boolean datatype.
+
+    """
+
     def __init__(self, left, right, operator, type_=None,
                  negate=None, modifiers=None):
         # allow compatibility with libraries that
@@ -2962,6 +2974,7 @@ class BinaryExpression(ColumnElement):
         self.operator = operator
         self.type = type_api.to_instance(type_)
         self.negate = negate
+        self._is_implicitly_boolean = operators.is_boolean(operator)
 
         if modifiers is None:
             self.modifiers = {}
@@ -3066,6 +3079,10 @@ class Grouping(ColumnElement):
     def self_group(self, against=None):
         return self
 
+    @util.memoized_property
+    def _is_implicitly_boolean(self):
+        return self.element._is_implicitly_boolean
+
     @property
     def _key_label(self):
         return self._label
@@ -3551,6 +3568,10 @@ class Label(ColumnElement):
         return self.__class__, (self.name, self._element, self._type)
 
     @util.memoized_property
+    def _is_implicitly_boolean(self):
+        return self.element._is_implicitly_boolean
+
+    @util.memoized_property
     def _allow_label_resolve(self):
         return self.element._allow_label_resolve
 
diff --git a/lib/sqlalchemy/sql/operators.py b/lib/sqlalchemy/sql/operators.py
index fd65979c5..a9f4e3e3e 100644
--- a/lib/sqlalchemy/sql/operators.py
+++ b/lib/sqlalchemy/sql/operators.py
@@ -1315,6 +1315,12 @@ def is_natural_self_precedent(op):
     return op in _natural_self_precedent or \
         isinstance(op, custom_op) and op.natural_self_precedent
 
+_booleans = (inv, istrue, isfalse, and_, or_)
+
+
+def is_boolean(op):
+    return is_comparison(op) or op in _booleans
+
 _mirror = {
     gt: lt,
     ge: le,
diff --git a/lib/sqlalchemy/sql/selectable.py b/lib/sqlalchemy/sql/selectable.py
index 04f6c086d..5b665829b 100644
--- a/lib/sqlalchemy/sql/selectable.py
+++ b/lib/sqlalchemy/sql/selectable.py
@@ -3576,6 +3576,7 @@ class Select(HasPrefixes, HasSuffixes, GenerativeSelect):
 class ScalarSelect(Generative, Grouping):
     _from_objects = []
     _is_from_container = True
+    _is_implicitly_boolean = False
 
     def __init__(self, element):
         self.element = element