Badge: Access to underlying type

Problem:
- Access to the underlying type is not provided. This limits
  metaprogramming and usage in function templates.

Solution:
- Provide public access to the underlying type.
- Add test to ensure the underlying type is accessible.

AK/Badge.h

@@ -30,6 +30,10 @@ namespace AK {
 
 template<typename T>
 class Badge {
+public:
+    using Type = T;
+
+private:
     friend T;
     constexpr Badge() = default;
 

AK/Tests/CMakeLists.txt

@@ -3,6 +3,7 @@ set(AK_TEST_SOURCES
     TestAnyOf.cpp
     TestArray.cpp
     TestAtomic.cpp
+    TestBadge.cpp
     TestBase64.cpp
     TestBinarySearch.cpp
     TestBitmap.cpp
@@ -18,8 +19,8 @@ set(AK_TEST_SOURCES
     TestFormat.cpp
     TestHashFunctions.cpp
     TestHashMap.cpp
-    TestIndexSequence.cpp
     TestIPv4Address.cpp
+    TestIndexSequence.cpp
     TestJSON.cpp
     TestLexicalPath.cpp
     TestMACAddress.cpp
@@ -39,8 +40,8 @@ set(AK_TEST_SOURCES
     TestStringUtils.cpp
     TestStringView.cpp
     TestTrie.cpp
-    TestTypedTransfer.cpp
     TestTypeTraits.cpp
+    TestTypedTransfer.cpp
     TestURL.cpp
     TestUtf8.cpp
     TestVector.cpp

AK/Tests/TestBadge.cpp

@@ -0,0 +1,36 @@
+/*
+ * Copyright (c) 2021, the SerenityOS developers.
+ * All rights reserved.
+ *
+ * Redistribution and use in source and binary forms, with or without
+ * modification, are permitted provided that the following conditions are met:
+ *
+ * 1. Redistributions of source code must retain the above copyright notice, this
+ *    list of conditions and the following disclaimer.
+ *
+ * 2. Redistributions in binary form must reproduce the above copyright notice,
+ *    this list of conditions and the following disclaimer in the documentation
+ *    and/or other materials provided with the distribution.
+ *
+ * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS"
+ * AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
+ * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE
+ * DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT HOLDER OR CONTRIBUTORS BE LIABLE
+ * FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
+ * DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR
+ * SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER
+ * CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY,
+ * OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
+ * OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
+ */
+
+#include <AK/TestSuite.h>
+
+#include <AK/Badge.h>
+
+TEST_CASE(should_provide_underlying_type)
+{
+    static_assert(AK::IsSame<int, Badge<int>::Type>::value);
+}
+
+TEST_MAIN(Badge)