[Util] Add is_allocator concept

Without this concept, compiler would pick the templated
perfect-forwarding constructor of `doubly_linked_list`
over its copy constructor for `non const` arguments.

Author: alir3zakh

include/mutable/util/ADT.hpp

@@ -365,7 +365,7 @@ struct dyn_array
 /** Implements a doubly-linked list with an overhead of just a single pointer per element. */
 template<
     typename T,
-    typename Allocator = malloc_allocator
+    is_allocator Allocator = malloc_allocator
 >
 struct doubly_linked_list
 {
@@ -459,7 +459,7 @@ struct doubly_linked_list
     /*----- Constructors & Destructor --------------------------------------------------------------------------------*/
     doubly_linked_list() : doubly_linked_list(allocator_type()) { }
 
-    template<typename A = allocator_type>
+    template<is_allocator A = allocator_type>
     explicit doubly_linked_list(A &&allocator)
         : allocator_(std::forward<A>(allocator))
     { }

include/mutable/util/allocator_base.hpp

@@ -1,11 +1,18 @@
 #pragma once
 
 #include <cstdint>
+#include <memory>
 #include <type_traits>
 
 
 namespace m {
 
+template <typename T>
+concept is_allocator = requires (T t, size_t size, size_t alignment, void *ptr) {
+    { t.allocate(size, alignment) } -> std::same_as<void*>;
+    { t.deallocate(ptr, size) } -> std::same_as<void>;
+};
+
 template<typename Actual>
 struct allocator
 {

unittest/util/ADTTest.cpp

@@ -592,10 +592,11 @@ TEST_CASE("doubly_linked_list", "[core][util]")
 
         SECTION("copy c'tor")
         {
-            const dlint L1(vec.begin(), vec.end());
+            dlint L1(vec.begin(), vec.end());
+            dlint& L1_ref = L1;
 
             dlint L2(L1);
-            dlint L3 = dlint(L1);
+            dlint L3 = dlint(L1_ref);
 
             CHECK_LIST(L1, { 42, 13, 73 });
             CHECK_LIST(L2, { 42, 13, 73 });
@@ -617,7 +618,7 @@ TEST_CASE("doubly_linked_list", "[core][util]")
 
         SECTION("= operator")
         {
-            const dlint L1(vec.begin(), vec.end());
+            dlint L1(vec.begin(), vec.end());
             dlint L2;
 
             L2 = dlint(L1);