[flat_set] Add a const fm.keys() accessor

As proposed in P2767R0.
Also, fix a unit test that was apparently too sneaky for C++23;
I didn't look far into what was going wrong with it.

include/sg14/flat_set.h

@@ -501,6 +501,10 @@ class flat_set {
     Compare key_comp() const { return compare_; }
     Compare value_comp() const { return compare_; }
 
+    const KeyContainer& keys() const {
+        return c_;
+    }
+
     iterator find(const Key& t) {
         auto it = this->lower_bound(t);
         if (it == this->end() || compare_(t, *it)) {

test/flat_map_test.cpp

@@ -714,13 +714,14 @@ TEST(flat_map, Iterators)
     EXPECT_EQ(fm.end(), fm.cend());
     EXPECT_EQ(fm.rbegin(), fm.crbegin());
     EXPECT_EQ(fm.rend(), fm.crend());
-    std::vector<std::pair<int, char>> underlying = {{1,'1'}, {2,'2'}, {3,'3'}, {4,'4'}};
-    std::vector<std::pair<const int&, char&>> expected(underlying.begin(), underlying.end());
-    std::vector<std::pair<const int&, const char&>> cexpected(underlying.begin(), underlying.end());
-    EXPECT_TRUE(std::equal(fm.begin(), fm.end(), expected.begin(), expected.end()));
-    EXPECT_TRUE(std::equal(fm.cbegin(), fm.cend(), cexpected.begin(), cexpected.end()));
-    EXPECT_TRUE(std::equal(fm.rbegin(), fm.rend(), expected.rbegin(), expected.rend()));
-    EXPECT_TRUE(std::equal(fm.crbegin(), fm.crend(), cexpected.rbegin(), cexpected.rend()));
+    std::vector<std::pair<int, char>> expected = {{1,'1'}, {2,'2'}, {3,'3'}, {4,'4'}};
+    auto eq = [](auto&& p1, auto&& p2) {
+        return (p1.first == p2.first) && (p1.second == p2.second);
+    };
+    EXPECT_TRUE(std::equal(fm.begin(), fm.end(), expected.begin(), expected.end(), eq));
+    EXPECT_TRUE(std::equal(fm.cbegin(), fm.cend(), expected.cbegin(), expected.cend(), eq));
+    EXPECT_TRUE(std::equal(fm.rbegin(), fm.rend(), expected.rbegin(), expected.rend(), eq));
+    EXPECT_TRUE(std::equal(fm.crbegin(), fm.crend(), expected.crbegin(), expected.crend(), eq));
 }
 
 TEST(flat_map, ContainerAccessors)

test/flat_set_test.cpp

@@ -322,3 +322,16 @@ TEST(flat_set, Iterators)
     EXPECT_TRUE(std::equal(fs.rbegin(), fs.rend(), expected.rbegin(), expected.rend()));
     EXPECT_TRUE(std::equal(fs.crbegin(), fs.crend(), expected.rbegin(), expected.rend()));
 }
+
+TEST(flat_set, ContainerAccessors)
+{
+    // The C++23 flat_set provides only `ctr = fs.extract()` and `fs.replace(ctr)`,
+    // but the SG14 version provides direct access to the container in place.
+    sg14::flat_set<int> fs = {3, 1, 4, 2};
+    const auto& cfs = fs;
+    static_assert(std::is_same<decltype(fs.keys()), const std::vector<int>&>::value, "");
+    static_assert(std::is_same<decltype(cfs.keys()), const std::vector<int>&>::value, "");
+    std::vector<int> expected_keys = {1, 2, 3, 4};
+    EXPECT_EQ(fs.keys(), expected_keys);
+    EXPECT_EQ(cfs.keys(), expected_keys);
+}