Fix frozen::make_set for the case of N = 0

Thanks to @cbeck88 for spotting the issue.

Author: serge-sans-paille-qb

include/frozen/bits/basic_types.h

@@ -33,6 +33,9 @@ namespace frozen {
 
 namespace bits {
 
+// used as a fake argument for frozen::make_set and frozen::make_map in the case of N=0
+struct ignored_arg {};
+
 template <class T, std::size_t N>
 class cvector {
   T data [N] = {}; // zero-initialization for scalar type T, default-initialized otherwise

include/frozen/map.h

@@ -66,8 +66,6 @@ template <class Comparator> class CompareKey {
   }
 };
 
-struct ignored_arg {};
-
 } // namespace impl
 
 template <class Key, class Value, std::size_t N, class Compare = std::less<Key>>
@@ -249,7 +247,7 @@ class map<Key, Value, 0, Compare> {
 };
 
 template <typename T, typename U>
-constexpr auto make_map(impl::ignored_arg = {}/* for consistency with the initializer below for N = 0*/) {
+constexpr auto make_map(bits::ignored_arg = {}/* for consistency with the initializer below for N = 0*/) {
   return map<T, U, 0>{};
 }
 

include/frozen/set.h

@@ -131,7 +131,7 @@ template <class Key, std::size_t N, class Compare = std::less<Key>> class set {
 };
 
 template <class Key, class Compare> class set<Key, 0, Compare> {
-  using container_type = bits::carray<Key, 1>; // just for the type definitions
+  using container_type = bits::carray<Key, 0>; // just for the type definitions
   Compare const compare_;
 
 public:
@@ -194,11 +194,17 @@ template <class Key, class Compare> class set<Key, 0, Compare> {
   constexpr const_reverse_iterator crend() const { return nullptr; }
 };
 
+template <typename T>
+constexpr auto make_set(bits::ignored_arg = {}/* for consistency with the initializer below for N = 0*/) {
+  return set<T, 0>{};
+}
+
 template <typename T, std::size_t N>
 constexpr auto make_set(const T (&args)[N]) {
   return set<T, N>(args);
 }
 
+
 } // namespace frozen
 
 #endif

tests/test_set.cpp

@@ -235,6 +235,24 @@ TEST_CASE("frozen::set <> frozen::make_set", "[set]") {
     for (auto v : frozen_set)
       REQUIRE(frozen_set2.count(v));
   }
+
+  constexpr frozen::set<short, 0> frozen_empty_set = {};
+  constexpr auto frozen_empty_set2 = frozen::make_set<short>();
+  constexpr auto frozen_empty_set3 = frozen::make_set<short>({});
+
+  SECTION("checking empty set") {
+    REQUIRE(frozen_empty_set.empty());
+    REQUIRE(frozen_empty_set.size() == 0);
+    REQUIRE(frozen_empty_set.begin() == frozen_empty_set.end());
+
+    REQUIRE(frozen_empty_set2.empty());
+    REQUIRE(frozen_empty_set2.size() == 0);
+    REQUIRE(frozen_empty_set2.begin() == frozen_empty_set2.end());
+
+    REQUIRE(frozen_empty_set3.empty());
+    REQUIRE(frozen_empty_set3.size() == 0);
+    REQUIRE(frozen_empty_set3.begin() == frozen_empty_set3.end());
+  }
 }
 
 TEST_CASE("frozen::set constexpr", "[set]") {