FIX: Member form validator fix.

src/Security/Member.php

@@ -15,6 +15,7 @@
 use SilverStripe\Core\Injector\Injector;
 use SilverStripe\Dev\TestMailer;
 use SilverStripe\Forms\CheckboxField;
+use SilverStripe\Forms\CompositeValidator;
 use SilverStripe\Forms\ConfirmedPasswordField;
 use SilverStripe\Forms\DropdownField;
 use SilverStripe\Forms\FieldList;
@@ -696,6 +697,20 @@ public function getValidator()
         return $validator;
     }
 
+    public function getCMSCompositeValidator(): CompositeValidator
+    {
+        // Add the member validator before extension point, so it's much easier to customise this
+        // via an extension
+        $this->beforeExtending(
+            'updateCMSCompositeValidator',
+            function (CompositeValidator $compositeValidator): void {
+                $memberValidator = $this->getValidator();
+                $compositeValidator->addValidator($memberValidator);
+            }
+        );
+
+        return parent::getCMSCompositeValidator();
+    }
 
     /**
      * Temporarily act as the specified user, limited to a $callback, but

tests/php/Core/Validation/MemberValidatorTest.php

@@ -0,0 +1,21 @@
+<?php
+
+namespace SilverStripe\Core\Tests\Validation;
+
+use SilverStripe\Dev\SapphireTest;
+use SilverStripe\Security\Member;
+use SilverStripe\Security\Member_Validator;
+
+class MemberValidatorTest extends SapphireTest
+{
+    protected $usesDatabase = false;
+
+    public function testMemberValidator(): void
+    {
+        $member = new Member();
+        $compositeValidator = $member->getCMSCompositeValidator();
+
+        $memberValidators = $compositeValidator->getValidatorsByType(Member_Validator::class);
+        $this->assertCount(1, $memberValidators, 'We expect exactly one member validator');
+    }
+}