Fixed #28391 -- Fixed Cast() with CharField and max_length on MySQL.

Thanks Tim Graham for the review.

django/db/backends/base/features.py

@@ -234,6 +234,9 @@ class BaseDatabaseFeatures:
     # Does the backend support indexing a TextField?
     supports_index_on_text_field = True
 
+    # Does the backend support CAST with precision?
+    supports_cast_with_precision = True
+
     def __init__(self, connection):
         self.connection = connection
 

django/db/backends/sqlite3/features.py

@@ -30,6 +30,7 @@ class DatabaseFeatures(BaseDatabaseFeatures):
     can_clone_databases = True
     supports_temporal_subtraction = True
     ignores_table_name_case = True
+    supports_cast_with_precision = False
 
     @cached_property
     def uses_savepoints(self):

django/db/models/fields/__init__.py

@@ -607,13 +607,16 @@ class Field(RegisterLookupMixin):
         self.run_validators(value)
         return value
 
+    def db_type_parameters(self, connection):
+        return DictWrapper(self.__dict__, connection.ops.quote_name, 'qn_')
+
     def db_check(self, connection):
         """
         Return the database column check constraint for this field, for the
         provided connection. Works the same way as db_type() for the case that
         get_internal_type() does not map to a preexisting model field.
         """
-        data = DictWrapper(self.__dict__, connection.ops.quote_name, "qn_")
+        data = self.db_type_parameters(connection)
         try:
             return connection.data_type_check_constraints[self.get_internal_type()] % data
         except KeyError:
@@ -639,7 +642,7 @@ class Field(RegisterLookupMixin):
         # mapped to one of the built-in Django field types. In this case, you
         # can implement db_type() instead of get_internal_type() to specify
         # exactly which wacky database column type you want to use.
-        data = DictWrapper(self.__dict__, connection.ops.quote_name, "qn_")
+        data = self.db_type_parameters(connection)
         try:
             return connection.data_types[self.get_internal_type()] % data
         except KeyError:

django/db/models/functions/base.py

@@ -10,7 +10,7 @@ class Cast(Func):
     template = '%(function)s(%(expressions)s AS %(db_type)s)'
 
     mysql_types = {
-        fields.CharField: 'char',
+        fields.CharField: 'char(%(max_length)s)',
         fields.IntegerField: 'signed integer',
         fields.BigIntegerField: 'signed integer',
         fields.SmallIntegerField: 'signed integer',
@@ -31,7 +31,8 @@ class Cast(Func):
         extra_context = {}
         output_field_class = type(self.output_field)
         if output_field_class in self.mysql_types:
-            extra_context['db_type'] = self.mysql_types[output_field_class]
+            data = self.output_field.db_type_parameters(connection)
+            extra_context['db_type'] = self.mysql_types[output_field_class] % data
         return self.as_sql(compiler, connection, **extra_context)
 
     def as_postgresql(self, compiler, connection):

tests/db_functions/test_cast.py

@@ -1,7 +1,7 @@
 from django.db import models
 from django.db.models.expressions import Value
 from django.db.models.functions import Cast
-from django.test import TestCase
+from django.test import TestCase, ignore_warnings, skipUnlessDBFeature
 
 from .models import Author
 
@@ -19,6 +19,13 @@ class CastTests(TestCase):
         numbers = Author.objects.annotate(cast_string=Cast('age', models.CharField(max_length=255)),)
         self.assertEqual(numbers.get().cast_string, '1')
 
+    # Silence "Truncated incorrect CHAR(1) value: 'Bob'".
+    @ignore_warnings(module='django.db.backends.mysql.base')
+    @skipUnlessDBFeature('supports_cast_with_precision')
+    def test_cast_to_char_field_with_max_length(self):
+        names = Author.objects.annotate(cast_string=Cast('name', models.CharField(max_length=1)))
+        self.assertEqual(names.get().cast_string, 'B')
+
     def test_cast_to_integer(self):
         for field_class in (
             models.IntegerField,