LWG-3810: CTAD for basic_format_args (#3421)

Co-authored-by: Stephan T. Lavavej <stl@nuwen.net> Co-authored-by: Casey Carter <Casey@Carter.net>
microsoft · Feb 14, 2023 · bf64ffc · bf64ffc
1 parent aee02b5
commit bf64ffc
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Showing 2 changed files with 11 additions and 0 deletions.
diff --git a/stl/inc/format b/stl/inc/format
@@ -1991,6 +1991,9 @@ private:
     const _Format_arg_index* _Index_array = nullptr;
 };
 
+template <class _Context, class... _Args>
+basic_format_args(_Format_arg_store<_Context, _Args...>) -> basic_format_args<_Context>;
+
 // _Lazy_locale is used instead of a std::locale so that the locale is only
 // constructed when needed, and is never constructed if the format string does not
 // contain locale-sensitive format specifiers. Note that this means that a new locale

diff --git a/tests/std/tests/P0645R10_text_formatting_args/test.cpp b/tests/std/tests/P0645R10_text_formatting_args/test.cpp
@@ -229,11 +229,19 @@ void test_visit_monostate() {
     assert(visit_format_arg(visitor<Context>, basic_format_arg<Context>()) == Arg_type::none);
 }
 
+template <class Context>
+void test_lwg3810() {
+    [[maybe_unused]] auto args_store = make_format_args<Context>(1, 2, 3);
+    static_assert(same_as<decltype(basic_format_args{args_store}), basic_format_args<Context>>);
+}
+
 int main() {
     test_basic_format_arg<format_context>();
     test_basic_format_arg<wformat_context>();
     test_format_arg_store<format_context>();
     test_format_arg_store<wformat_context>();
     test_visit_monostate<format_context>();
     test_visit_monostate<wformat_context>();
+    test_lwg3810<format_context>();
+    test_lwg3810<wformat_context>();
 }