id	summary	reporter	owner	description	type	status	component	version	severity	resolution	keywords	cc	stage	has_patch	needs_docs	needs_tests	needs_better_patch	easy	ui_ux
37470	NegatedExpression doesn't handle FullResultSet, so negating an always-true condition evaluates to true.	maiload	maiload	"`NegatedExpression.as_sql()` handles `EmptyResultSet` raised by the wrapped expression, but not `FullResultSet`. When the wrapped condition matches everything, `FullResultSet` propagates un-negated and callers treat `NOT <always true>` as always true.

One visible consequence is that a `CheckConstraint` using a negated lookup is not enforced by model validation for `NULL` values, while the database does enforce it:

{{{#!python
from django.db import models
from django.db.models import F
from django.db.models.lookups import IsNull


class Item(models.Model):
    value = models.IntegerField(null=True)

    class Meta:
        constraints = [
            models.CheckConstraint(
                condition=~IsNull(F(""value""), True),
                name=""value_not_null"",
            ),
        ]
}}}

{{{#!python
Item(value=None).validate_constraints()  # Expected ValidationError, nothing is raised.
Item(value=None).save()  # The database rejects it: IntegrityError.
}}}

The equivalent `~Q(value__isnull=True)` constraint raises `ValidationError` as expected.

The same root cause affects queries:

{{{#!python
from django.db.models import BooleanField, ExpressionWrapper, Q

always_false = ~ExpressionWrapper(~Q(pk__in=[]), output_field=BooleanField())
Item.objects.filter(always_false)            # Expected: no rows. Actual: all rows.
Item.objects.annotate(x=always_false)        # Expected: x=False. Actual: x=True.
}}}

{{{#!python
always_false = ~ExpressionWrapper(~Q(pk__in=[]), output_field=BooleanField())
Item.objects.filter(always_false)            # Expected: no rows. Actual: all rows.
Item.objects.annotate(x=always_false)        # Expected: x=False. Actual: x=True.
}}}

The `CheckConstraint` case is a regression in 3434dbd39d373df7193ad006b970c09c1a909ea3 (#34754, backported to 4.2.5), which made `IsNull(Value(None), True)` raise `FullResultSet`. On its parent commit (2b582387d5) `validate_constraints()` correctly raises `ValidationError`. The queryset cases have been incorrect since `FullResultSet` was introduced in 76e37513e22f4d9a01c7f15eee36fe44388e6670 (refs #33374), shortly after `NegatedExpression` was added for #16211.

Adding an `except FullResultSet: raise EmptyResultSet` branch to `NegatedExpression.as_sql()` fixes it: the reproduction tests pass and the full test suite passes on SQLite with that change. I haven't tested other backends yet. I'd like to submit a PR with a regression test.

Reproduced on main (b501b8493f) with SQLite.

I used Claude Code to help explore the codebase and draft the reproduction tests. I verified the behavior, the regression commit, and the test results myself."	Bug	assigned	Database layer (models, ORM)	dev	Normal		NegatedExpression FullResultSet CheckConstraint	maiload	Unreviewed	0	0	0	0	0	0