Turn off the is_literal flag when proxying literal_column() to Label

Fixed a series of quoting issues which all stemmed from the concept of the
:func:`.literal_column` construct, which when being "proxied" through a
subquery to be referred towards by a label that matches its text, the label
would not have quoting rules applied to it, even if the string in the
:class:`.Label` were set up as a :class:`.quoted_name` construct.  Not
applying quoting to the text of the :class:`.Label` is a bug because this
text is strictly a SQL identifier name and not a SQL expression, and the
string should not have quotes embedded into it already unlike the
:func:`.literal_column` which it may be applied towards.   The existing
behavior of a non-labeled :func:`.literal_column` being propagated as is on
the outside of a subquery is maintained in order to help with manual
quoting schemes, although it's not clear if valid SQL can be generated for
such a construct in any case.

Fixes: #4730
Change-Id: I300941f27872fc4298c74a1d1ed65aef1a5cdd82

3 files changed, 22 insertions, 6 deletions

diff --git a/lib/sqlalchemy/sql/compiler.py b/lib/sqlalchemy/sql/compiler.py
index 6fcf1a524..7922054f8 100644
--- a/lib/sqlalchemy/sql/compiler.py
+++ b/lib/sqlalchemy/sql/compiler.py
@@ -851,6 +851,8 @@ class SQLCompiler(Compiled):
             )
 
         if is_literal:
+            # note we are not currently accommodating for
+            # literal_column(quoted_name('ident', True)) here
             name = self.escape_literal_column(name)
         else:
             name = self.preparer.quote(name)
diff --git a/lib/sqlalchemy/sql/elements.py b/lib/sqlalchemy/sql/elements.py
index a333303ec..cc57e58e5 100644
--- a/lib/sqlalchemy/sql/elements.py
+++ b/lib/sqlalchemy/sql/elements.py
@@ -3895,7 +3895,9 @@ class Label(roles.LabeledColumnExprRole, ColumnElement):
 
     def _make_proxy(self, selectable, name=None, **kw):
         e = self.element._make_proxy(
-            selectable, name=name if name else self.name
+            selectable,
+            name=name if name else self.name,
+            disallow_is_literal=True,
         )
         e._proxies.append(self)
         if self._type is not None:
@@ -4029,7 +4031,6 @@ class ColumnClause(roles.LabeledColumnExprRole, Immutable, ColumnElement):
             :ref:`sqlexpression_literal_column`
 
         """
-
         self.key = self.name = text
         self.table = _selectable
         self.type = type_api.to_instance(type_)
@@ -4158,11 +4159,25 @@ class ColumnClause(roles.LabeledColumnExprRole, Immutable, ColumnElement):
         name=None,
         attach=True,
         name_is_truncatable=False,
+        disallow_is_literal=False,
         **kw
     ):
-        # propagate the "is_literal" flag only if we are keeping our name,
-        # otherwise its considered to be a label
-        is_literal = self.is_literal and (name is None or name == self.name)
+        # the "is_literal" flag normally should never be propagated; a proxied
+        # column is always a SQL identifier and never the actual expression
+        # being evaluated. however, there is a case where the "is_literal" flag
+        # might be used to allow the given identifier to have a fixed quoting
+        # pattern already, so maintain the flag for the proxy unless a
+        # :class:`.Label` object is creating the proxy.  See [ticket:4730].
+        is_literal = (
+            not disallow_is_literal
+            and self.is_literal
+            and (
+                # note this does not accommodate for quoted_name differences
+                # right now
+                name is None
+                or name == self.name
+            )
+        )
         c = self._constructor(
             coercions.expect(roles.TruncatedLabelRole, name or self.name)
             if name_is_truncatable
diff --git a/lib/sqlalchemy/sql/selectable.py b/lib/sqlalchemy/sql/selectable.py
index 014c782d0..fbc33ea9b 100644
--- a/lib/sqlalchemy/sql/selectable.py
+++ b/lib/sqlalchemy/sql/selectable.py
@@ -3842,7 +3842,6 @@ class Select(HasPrefixes, HasSuffixes, GenerativeSelect):
                     key = c.anon_label
             else:
                 key = None
-
             c._make_proxy(self, key=key, name=name, name_is_truncatable=True)
 
     def _refresh_for_new_column(self, column):