Implement P2905R2 Runtime Format Strings (#4196)

Co-authored-by: Stephan T. Lavavej <[email protected]>

stl/inc/format

@@ -3688,18 +3688,20 @@ _EXPORT_STD using format_args  = basic_format_args<format_context>;
 _EXPORT_STD using wformat_args = basic_format_args<wformat_context>;
 
 _EXPORT_STD template <class _Context = format_context, class... _Args>
-_NODISCARD auto make_format_args(_Args&&... _Vals) {
-    static_assert((_Formattable_with<remove_cvref_t<_Args>, _Context> && ...),
+_NODISCARD auto make_format_args(_Args&... _Vals) {
+    // TRANSITION, should cite the new working draft
+    static_assert((_Formattable_with<remove_const_t<_Args>, _Context> && ...),
         "Cannot format an argument. To make type T formattable, provide a formatter<T> specialization. "
-        "See N4950 [format.arg.store]/2 and [formatter.requirements].");
+        "See N4964 [format.arg.store]/2 (along with modification in P2905R2) and [formatter.requirements].");
     return _Format_arg_store<_Context, _Args...>{_Vals...};
 }
 
 _EXPORT_STD template <class... _Args>
-_NODISCARD auto make_wformat_args(_Args&&... _Vals) {
-    static_assert((_Formattable_with<remove_cvref_t<_Args>, wformat_context> && ...),
+_NODISCARD auto make_wformat_args(_Args&... _Vals) {
+    // TRANSITION, should cite the new working draft
+    static_assert((_Formattable_with<remove_const_t<_Args>, wformat_context> && ...),
         "Cannot format an argument. To make type T formattable, provide a formatter<T> specialization. "
-        "See N4950 [format.arg.store]/2 and [formatter.requirements].");
+        "See N4964 [format.arg.store]/2 (along with modification in P2905R2) and [formatter.requirements].");
     return _Format_arg_store<wformat_context, _Args...>{_Vals...};
 }
 

stl/inc/ostream

@@ -1244,11 +1244,9 @@ void _Print_impl(const _Add_newline _Add_nl, ostream& _Ostr, const format_string
 
     if constexpr (_Has_format_args) {
         if constexpr (_STD _Is_ordinary_literal_encoding_utf8()) {
-            _STD _Vprint_unicode_impl(
-                _Add_nl, _Ostr, _Fmt.get(), _STD make_format_args(_STD forward<_Types>(_Args)...));
+            _STD _Vprint_unicode_impl(_Add_nl, _Ostr, _Fmt.get(), _STD make_format_args(_Args...));
         } else {
-            _STD _Vprint_nonunicode_impl(
-                _Add_nl, _Ostr, _Fmt.get(), _STD make_format_args(_STD forward<_Types>(_Args)...));
+            _STD _Vprint_nonunicode_impl(_Add_nl, _Ostr, _Fmt.get(), _STD make_format_args(_Args...));
         }
     } else {
         const string _Unescaped_str{_Unescape_braces(_Add_nl, _Fmt.get())};

stl/inc/print

@@ -101,11 +101,9 @@ void _Print_impl(
 
     if constexpr (_Has_format_args) {
         if constexpr (_STD _Is_ordinary_literal_encoding_utf8()) {
-            _STD _Vprint_unicode_impl(
-                _Add_nl, _Stream, _Fmt.get(), _STD make_format_args(_STD forward<_Types>(_Args)...));
+            _STD _Vprint_unicode_impl(_Add_nl, _Stream, _Fmt.get(), _STD make_format_args(_Args...));
         } else {
-            _STD _Vprint_nonunicode_impl(
-                _Add_nl, _Stream, _Fmt.get(), _STD make_format_args(_STD forward<_Types>(_Args)...));
+            _STD _Vprint_nonunicode_impl(_Add_nl, _Stream, _Fmt.get(), _STD make_format_args(_Args...));
         }
     } else {
         const string _Unescaped_str{_Unescape_braces(_Add_nl, _Fmt.get())};

stl/inc/yvals_core.h

@@ -307,6 +307,7 @@
 // P2711R1 Making Multi-Param Constructors Of Views explicit
 // P2736R2 Referencing The Unicode Standard
 // P2770R0 Stashing Stashing Iterators For Proper Flattening
+// P2905R2 Runtime Format Strings
 // P2909R4 Fix Formatting Of Code Units As Integers
 
 // _HAS_CXX20 indirectly controls:

tests/libcxx/expected_results.txt

@@ -308,10 +308,6 @@ std/language.support/support.limits/support.limits.general/utility.version.compi
 std/language.support/support.limits/support.limits.general/expected.version.compile.pass.cpp FAIL
 std/language.support/support.limits/support.limits.general/mdspan.version.compile.pass.cpp FAIL
 
-# P2905R2 Runtime Format Strings
-std/utilities/format/format.arguments/format.arg.store/make_format_args.pass.cpp FAIL
-std/utilities/format/format.arguments/format.arg.store/make_wformat_args.pass.cpp FAIL
-
 # P2918R2 Runtime Format Strings II
 std/utilities/format/format.fmt.string/ctor.runtime-format-string.pass.cpp FAIL
 std/utilities/format/format.functions/format.locale.runtime_format.pass.cpp FAIL

tests/std/include/test_format_support.hpp

@@ -161,9 +161,9 @@ struct VFormatFn {
     template <class... Args>
     [[nodiscard]] auto operator()(const std::basic_string_view<CharT> str, Args&&... args) const {
         if constexpr (std::same_as<CharT, char>) {
-            return std::vformat(str, std::make_format_args(std::forward<Args>(args)...));
+            return std::vformat(str, std::make_format_args(args...));
         } else {
-            return std::vformat(str, std::make_wformat_args(std::forward<Args>(args)...));
+            return std::vformat(str, std::make_wformat_args(args...));
         }
     }
 };

tests/std/tests/P0355R7_calendars_and_time_zones_formatting/test.cpp

@@ -213,11 +213,11 @@ bool test_parse_chrono_format_specs() {
 }
 
 template <class charT, class... Args>
-auto make_testing_format_args(Args&&... vals) {
+auto make_testing_format_args(Args&&... vals) { // references to temporaries are risky, see P2905R2; we'll be careful
     if constexpr (is_same_v<charT, wchar_t>) {
-        return make_wformat_args(forward<Args>(vals)...);
+        return make_wformat_args(vals...);
     } else {
-        return make_format_args(forward<Args>(vals)...);
+        return make_format_args(vals...);
     }
 }
 

tests/std/tests/P0645R10_text_formatting_args/test.cpp

@@ -248,7 +248,8 @@ void test_visit_monostate() {
 
 template <class Context>
 void test_lwg3810() {
-    [[maybe_unused]] auto args_store = make_format_args<Context>(1, 2, 3);
+    int args[]{1, 2, 3};
+    [[maybe_unused]] auto args_store = make_format_args<Context>(args[0], args[1], args[2]);
     static_assert(same_as<decltype(basic_format_args{args_store}), basic_format_args<Context>>);
 }
 
@@ -264,6 +265,54 @@ void test_lvalue_only_visitation() {
     visit_format_arg(lvalue_only_visitor{}, basic_format_arg<Context>{});
 }
 
+namespace detail {
+    constexpr bool permissive() {
+        return false;
+    }
+
+    template <class>
+    struct DependentBase {
+        static constexpr bool permissive() {
+            return true;
+        }
+    };
+
+    template <class T>
+    struct Derived : DependentBase<T> {
+        static constexpr bool test() {
+            return permissive();
+        }
+    };
+} // namespace detail
+constexpr bool is_permissive = detail::Derived<int>::test();
+
+template <class Context, class... Args>
+concept CanMakeFormatArgs = requires(Args&&... args) { make_format_args<Context>(static_cast<Args&&>(args)...); };
+
+// P2905R2 Runtime format strings (make make_(w)format_args only take lvalue references)
+template <class Context>
+void test_lvalue_reference_parameters() {
+    using char_type = Context::char_type;
+
+    static_assert(CanMakeFormatArgs<Context, int&, long long&, double&, char_type&, char_type*&, const char_type*&,
+        basic_string<char_type>&, basic_string_view<char_type>&>);
+    static_assert(
+        CanMakeFormatArgs<Context, const int&, const long long&, const double&, const char_type&, char_type* const&,
+            const char_type* const&, const basic_string<char_type>&, const basic_string_view<char_type>&>);
+
+    static_assert(CanMakeFormatArgs<Context, const int, const long long, const double, const char_type,
+        char_type* const, const char_type* const, const basic_string<char_type>, const basic_string_view<char_type>>);
+
+    static_assert(!CanMakeFormatArgs<Context, int>);
+    static_assert(!CanMakeFormatArgs<Context, long long>);
+    static_assert(!CanMakeFormatArgs<Context, double>);
+    static_assert(!CanMakeFormatArgs<Context, char_type>);
+    static_assert(!CanMakeFormatArgs<Context, char_type*>);
+    static_assert(!CanMakeFormatArgs<Context, const char_type*>);
+    static_assert(CanMakeFormatArgs<Context, basic_string<char_type>> == is_permissive);
+    static_assert(CanMakeFormatArgs<Context, basic_string_view<char_type>> == is_permissive);
+}
+
 int main() {
     test_basic_format_arg<format_context>();
     test_basic_format_arg<wformat_context>();
@@ -277,4 +326,7 @@ int main() {
 
     test_lvalue_only_visitation<format_context>();
     test_lvalue_only_visitation<wformat_context>();
+
+    test_lvalue_reference_parameters<format_context>();
+    test_lvalue_reference_parameters<wformat_context>();
 }

tests/std/tests/P0645R10_text_formatting_custom_formatting/test.cpp

@@ -15,11 +15,11 @@ using namespace std;
 
 // copied from the text_formatting_formatting test case
 template <class charT, class... Args>
-auto make_testing_format_args(Args&&... vals) {
+auto make_testing_format_args(Args&&... vals) { // references to temporaries are risky, see P2905R2; we'll be careful
     if constexpr (is_same_v<charT, wchar_t>) {
-        return make_wformat_args(forward<Args>(vals)...);
+        return make_wformat_args(vals...);
     } else {
-        return make_format_args(forward<Args>(vals)...);
+        return make_format_args(vals...);
     }
 }
 

tests/std/tests/P0645R10_text_formatting_formatting/test.cpp

@@ -63,11 +63,11 @@ template <class CharT, class Alloc = allocator<CharT>>
 using alternative_basic_string = basic_string<CharT, alternative_char_traits<CharT>, Alloc>;
 
 template <class charT, class... Args>
-auto make_testing_format_args(Args&&... vals) {
+auto make_testing_format_args(Args&&... vals) { // references to temporaries are risky, see P2905R2; we'll be careful
     if constexpr (is_same_v<charT, wchar_t>) {
-        return make_wformat_args(forward<Args>(vals)...);
+        return make_wformat_args(vals...);
     } else {
-        return make_format_args(forward<Args>(vals)...);
+        return make_format_args(vals...);
     }
 }
 

tests/std/tests/P0645R10_text_formatting_utf8/test.cpp

@@ -23,7 +23,8 @@ void test_multibyte_format_strings() {
     {
         try {
             // Bad fill character encoding: missing lead byte before \x9f
-            (void) vformat("{:\x9f\x8f\x88<10}"sv, make_format_args(42));
+            int arg = 42;
+            (void) vformat("{:\x9f\x8f\x88<10}"sv, make_format_args(arg));
             assert(false);
         } catch (const format_error&) {
         }

tests/std/tests/P2286R8_text_formatting_escaping/test.cpp

@@ -22,11 +22,11 @@ template <typename CharT>
 #define STR(Literal) (choose_literal<CharT>(Literal, L##Literal))
 
 template <class charT, class... Args>
-auto make_testing_format_args(Args&&... vals) {
+auto make_testing_format_args(Args&&... vals) { // references to temporaries are risky, see P2905R2; we'll be careful
     if constexpr (is_same_v<charT, wchar_t>) {
-        return make_wformat_args(forward<Args>(vals)...);
+        return make_wformat_args(vals...);
     } else {
-        return make_format_args(forward<Args>(vals)...);
+        return make_format_args(vals...);
     }
 }