[libc++][format] Add tests for flat_(|multi)map formatting (#124418)

These types 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 +10,8 @@
 #define TEST_STD_UTILITIES_FORMAT_FORMAT_RANGE_FORMAT_RANGE_FMTMAP_FORMAT_FUNCTIONS_TESTS_H
 
 #include <algorithm>
+#include <deque>
+#include <flat_map>
 #include <format>
 #include <map>
 #include <unordered_map>
@@ -241,10 +243,7 @@ void test_char_to_wchar(TestFunction check, ExceptionTest check_exception) {
 // Bool
 //
 template <class CharT, class TestFunction, class ExceptionTest>
-void test_bool(TestFunction check, ExceptionTest check_exception) {
-  // duplicates are stored in order of insertion
-  std::multimap<bool, int> input{{true, 42}, {false, 0}, {true, 1}};
-
+void test_bool(TestFunction check, ExceptionTest check_exception, auto&& input) {
   check(SV("{false: 0, true: 42, true: 1}"), SV("{}"), input);
   check(SV("{false: 0, true: 42, true: 1}^42"), SV("{}^42"), input);
   check(SV("{false: 0, true: 42, true: 1}^42"), SV("{:}^42"), input);
@@ -339,6 +338,17 @@ void test_bool(TestFunction check, ExceptionTest check_exception) {
       "The argument index value is too large for the number of arguments supplied", SV("{:^^{}:#>{}}"), input, 41);
 }
 
+template <class CharT, class TestFunction, class ExceptionTest>
+void test_bool(TestFunction check, ExceptionTest check_exception) {
+  // duplicates are stored in order of insertion
+  test_bool<CharT>(check, check_exception, std::multimap<bool, int>{{true, 42}, {false, 0}, {true, 1}});
+#if TEST_STD_VER >= 23
+  test_bool<CharT>(check,
+                   check_exception,
+                   std::flat_multimap<bool, int, std::less<bool>, std::deque<bool>>{{true, 42}, {false, 0}, {true, 1}});
+#endif
+}
+
 //
 // Integral
 //
@@ -442,6 +452,9 @@ 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
 }
 
 //