Add a niche to box

MODULE.bazel.lock

@@ -64,20 +64,20 @@
     "@@apple_support~//crosstool:setup.bzl%apple_cc_configure_extension": {
       "general": {
         "bzlTransitiveDigest": "PjIds3feoYE8SGbbIq2SFTZy3zmxeO2tQevJZNDo7iY=",
-        "usagesDigest": "+hz7IHWN6A1oVJJWNDB6yZRG+RYhF76wAYItpAeIUIg=",
+        "usagesDigest": "aLmqbvowmHkkBPve05yyDNGN7oh7QE9kBADr3QIZTZs=",
         "recordedFileInputs": {},
         "recordedDirentsInputs": {},
         "envVariables": {},
         "generatedRepoSpecs": {
-          "local_config_apple_cc_toolchains": {
-            "bzlFile": "@@apple_support~//crosstool:setup.bzl",
-            "ruleClassName": "_apple_cc_autoconf_toolchains",
-            "attributes": {}
-          },
           "local_config_apple_cc": {
             "bzlFile": "@@apple_support~//crosstool:setup.bzl",
             "ruleClassName": "_apple_cc_autoconf",
             "attributes": {}
+          },
+          "local_config_apple_cc_toolchains": {
+            "bzlFile": "@@apple_support~//crosstool:setup.bzl",
+            "ruleClassName": "_apple_cc_autoconf_toolchains",
+            "attributes": {}
           }
         },
         "recordedRepoMappingEntries": [
@@ -92,7 +92,7 @@
     "@@platforms//host:extension.bzl%host_platform": {
       "general": {
         "bzlTransitiveDigest": "xelQcPZH8+tmuOHVjL9vDxMnnQNMlwj0SlvgoqBkm4U=",
-        "usagesDigest": "pCYpDQmqMbmiiPI1p2Kd3VLm5T48rRAht5WdW0X2GlA=",
+        "usagesDigest": "meSzxn3DUCcYEhq4HQwExWkWtU4EjriRBQLsZN+Q0SU=",
         "recordedFileInputs": {},
         "recordedDirentsInputs": {},
         "envVariables": {},

asl/annotations.hpp

@@ -2,7 +2,6 @@
 
 #include "asl/config.hpp"
 
-
 #if ASL_COMPILER_CLANG_CL
     #define ASL_NO_UNIQUE_ADDRESS [[msvc::no_unique_address]]
 #elif ASL_COMPILER_CLANG

asl/box.hpp

@@ -16,15 +16,18 @@ class box
     ASL_NO_UNIQUE_ADDRESS Allocator m_alloc;
     
 public:
-    explicit constexpr box(T* ptr = nullptr)
+    explicit constexpr box(niche)
         requires default_constructible<Allocator>
-        : m_ptr{ptr}
+        : m_ptr{nullptr}
+        , m_alloc{}
     {}
-    
+
     constexpr box(T* ptr, Allocator alloc)
         : m_ptr{ptr}
         , m_alloc{ASL_MOVE(alloc)}
-    {}
+    {
+        ASL_ASSERT(m_ptr != nullptr);
+    }
 
     constexpr box(box&& other)
         : m_ptr{exchange(other.m_ptr, nullptr)}
@@ -77,6 +80,11 @@ public:
         ASL_ASSERT(m_ptr != nullptr);
         return m_ptr;
     }
+
+    constexpr bool operator==(niche) const
+    {
+        return m_ptr == nullptr;
+    }
 };
 
 template<is_object T, allocator Allocator = DefaultAllocator, typename... Args>

asl/meta.hpp

@@ -160,4 +160,13 @@ template<typename T, int N> struct _is_array_helper<T[N]> : true_type  {};
 
 template<typename T> concept is_array = _is_array_helper<T>::value;
 
+struct niche {};
+
+template<typename T>
+concept has_niche = constructible_from<T, niche> &&
+    requires (const T& value, niche n)
+    {
+        { value == n } -> same_as<bool>;
+    };
+
 } // namespace asl

asl/tests/box_tests.cpp

@@ -4,30 +4,20 @@
 #include "asl/tests/test_types.hpp"
 
 static_assert(sizeof(asl::box<int>) == sizeof(int*));
-static_assert(asl::default_constructible<asl::box<int>>);
 static_assert(!asl::copyable<asl::box<int>>);
 static_assert(asl::moveable<asl::box<int>>);
-static_assert(asl::default_constructible<asl::box<NonMoveConstructible>>);
+static_assert(asl::has_niche<asl::box<int>>);
 
 ASL_TEST(destructor)
 {
     bool d = false;
 
     {
-        asl::box<DestructorObserver> box2;
-    
-        {
-            auto box = asl::make_box<DestructorObserver>(&d);
-            ASL_TEST_ASSERT(!d);
-    
+        auto box = asl::make_box<DestructorObserver>(&d);
+        ASL_TEST_ASSERT(!d);
 
-            auto box3 = ASL_MOVE(box);
-            ASL_TEST_ASSERT(!d);
 
-            box2 = ASL_MOVE(box3);
-            ASL_TEST_ASSERT(!d);
-        }
-    
+        auto box3 = ASL_MOVE(box);
         ASL_TEST_ASSERT(!d);
     }