[libc++][format] Adds flat_map formatter.

This type should be formattable out-of-the-box. This patch validates
that is true.

Author: mordante

libcxx/test/std/utilities/format/format.formattable/concept.formattable.compile.pass.cpp

@@ -24,6 +24,7 @@
 #include <concepts>
 #include <deque>
 #include <filesystem>
+#include <flat_map>
 #include <format>
 #include <forward_list>
 #include <list>
@@ -242,6 +243,13 @@ void test_P2286() {
   assert_is_formattable<std::multiset<int>, CharT>();
   assert_is_formattable<std::multimap<int, int>, CharT>();
 
+#if TEST_STD_VER >= 23
+  // assert_is_formattable<std::flat_set<int>, CharT>();
+  assert_is_formattable<std::flat_map<int, int>, CharT>();
+  // assert_is_formattable<std::flat_multiset<int>, CharT>();
+  // assert_is_formattable<std::flat_multimap<int, int>, CharT>();
+#endif // TEST_STD_VER >= 2
+
   assert_is_formattable<std::unordered_set<int>, CharT>();
   assert_is_formattable<std::unordered_map<int, int>, CharT>();
   assert_is_formattable<std::unordered_multiset<int>, CharT>();

libcxx/test/std/utilities/format/format.range/format.range.fmtmap/format.functions.tests.h

@@ -10,6 +10,7 @@
 #define TEST_STD_UTILITIES_FORMAT_FORMAT_RANGE_FORMAT_RANGE_FMTMAP_FORMAT_FUNCTIONS_TESTS_H
 
 #include <algorithm>
+#include <flat_map>
 #include <format>
 #include <map>
 #include <unordered_map>
@@ -442,6 +443,9 @@ void test_int(TestFunction check, ExceptionTest check_exception, auto&& input) {
 template <class CharT, class TestFunction, class ExceptionTest>
 void test_int(TestFunction check, ExceptionTest check_exception) {
   test_int<CharT>(check, check_exception, std::map<int, int>{{1, -1}, {42, -42}, {-42, 42}});
+#if TEST_STD_VER >= 23
+  test_int<CharT>(check, check_exception, std::flat_map<int, int>{{1, -1}, {42, -42}, {-42, 42}});
+#endif
 }
 
 //