Fixed #30916 -- Added support for functional unique constraints.

Thanks Ian Foote and Mariusz Felisiak for reviews.

docs/ref/checks.txt

@@ -391,6 +391,8 @@ Models
 * **models.W042**: Auto-created primary key used when not defining a primary
   key type, by default ``django.db.models.AutoField``.
 * **models.W043**: ``<database>`` does not support indexes on expressions.
+* **models.W044**: ``<database>`` does not support unique constraints on
+  expressions.
 
 Security
 --------

docs/ref/contrib/postgres/indexes.txt

@@ -183,10 +183,10 @@ available from the ``django.contrib.postgres.indexes`` module.
 .. class:: OpClass(expression, name)
 
     An ``OpClass()`` expression represents the ``expression`` with a custom
-    `operator class`_ that can be used to define functional indexes. To use it,
-    you need to add ``'django.contrib.postgres'`` in your
-    :setting:`INSTALLED_APPS`. Set the ``name`` parameter to the name of the
-    `operator class`_.
+    `operator class`_ that can be used to define functional indexes or unique
+    constraints. To use it, you need to add ``'django.contrib.postgres'`` in
+    your :setting:`INSTALLED_APPS`. Set the ``name`` parameter to the name of
+    the `operator class`_.
 
     For example::
 
@@ -197,4 +197,18 @@ available from the ``django.contrib.postgres.indexes`` module.
 
     creates an index on ``Lower('username')`` using ``varchar_pattern_ops``.
 
+    Another example::
+
+        UniqueConstraint(
+            OpClass(Upper('description'), name='text_pattern_ops'),
+            name='upper_description_unique',
+        )
+
+    creates a unique constraint on ``Upper('description')`` using
+    ``text_pattern_ops``.
+
+    .. versionchanged:: 4.0
+
+        Support for functional unique constraints was added.
+
     .. _operator class: https://www.postgresql.org/docs/current/indexes-opclass.html

docs/ref/models/constraints.txt

@@ -69,10 +69,30 @@ constraint.
 ``UniqueConstraint``
 ====================
 
-.. class:: UniqueConstraint(*, fields, name, condition=None, deferrable=None, include=None, opclasses=())
+.. class:: UniqueConstraint(*expressions, fields=(), name=None, condition=None, deferrable=None, include=None, opclasses=())
 
     Creates a unique constraint in the database.
 
+``expressions``
+---------------
+
+.. attribute:: UniqueConstraint.expressions
+
+.. versionadded:: 4.0
+
+Positional argument ``*expressions`` allows creating functional unique
+constraints on expressions and database functions.
+
+For example::
+
+    UniqueConstraint(Lower('name').desc(), 'category', name='unique_lower_name_category')
+
+creates a unique constraint on the lowercased value of the ``name`` field in
+descending order and the ``category`` field in the default ascending order.
+
+Functional unique constraints have the same database restrictions as
+:attr:`Index.expressions`.
+
 ``fields``
 ----------
 

docs/releases/4.0.txt

@@ -28,6 +28,36 @@ The Django 3.2.x series is the last to support Python 3.6 and 3.7.
 What's new in Django 4.0
 ========================
 
+Functional unique constraints
+-----------------------------
+
+The new :attr:`*expressions <django.db.models.UniqueConstraint.expressions>`
+positional argument of
+:class:`UniqueConstraint() <django.db.models.UniqueConstraint>` enables
+creating functional unique constraints on expressions and database functions.
+For example::
+
+    from django.db import models
+    from django.db.models import UniqueConstraint
+    from django.db.models.functions import Lower
+
+
+    class MyModel(models.Model):
+        first_name = models.CharField(max_length=255)
+        last_name = models.CharField(max_length=255)
+
+        class Meta:
+            indexes = [
+                UniqueConstraint(
+                    Lower('first_name'),
+                    Lower('last_name').desc(),
+                    name='first_last_name_unique',
+                ),
+            ]
+
+Functional unique constraints are added to models using the
+:attr:`Meta.constraints <django.db.models.Options.constraints>` option.
+
 Minor features
 --------------