add support for statically named arguments with FMT_STRING

include/fmt/compile.h

@@ -191,24 +191,10 @@ constexpr const auto& get([[maybe_unused]] const T& first,
     return get<N - 1>(rest...);
 }
 
-constexpr int invalid_arg_index = -1;
-
-template <int N, typename T, typename... Args, typename Char>
-constexpr int get_arg_index_by_name(basic_string_view<Char> name) {
-  if constexpr (detail::is_statically_named_arg<T>()) {
-    if (name == T::name) return N;
-  }
-  if constexpr (sizeof...(Args) == 0) {
-    return invalid_arg_index;
-  } else {
-    return get_arg_index_by_name<N + 1, Args...>(name);
-  }
-}
-
 template <typename Char, typename... Args>
 constexpr int get_arg_index_by_name(basic_string_view<Char> name,
                                     type_list<Args...>) {
-  return get_arg_index_by_name<0, Args...>(name);
+  return get_arg_index_by_name<Args...>(name);
 }
 
 template <int N, typename> struct get_type_impl;

include/fmt/core.h

@@ -281,6 +281,16 @@
 #  define FMT_COMPILE_TIME_CHECKS 0
 #endif
 
+#ifndef FMT_USE_NONTYPE_TEMPLATE_PARAMETERS
+#  if defined(__cpp_nontype_template_args) &&                \
+      ((FMT_GCC_VERSION >= 903 && __cplusplus >= 201709L) || \
+       __cpp_nontype_template_args >= 201911L)
+#    define FMT_USE_NONTYPE_TEMPLATE_PARAMETERS 1
+#  else
+#    define FMT_USE_NONTYPE_TEMPLATE_PARAMETERS 0
+#  endif
+#endif
+
 // Enable minimal optimizations for more compact code in debug mode.
 FMT_GCC_PRAGMA("GCC push_options")
 #ifndef __OPTIMIZE__
@@ -991,6 +1001,7 @@ template <typename Char>
 inline void init_named_args(named_arg_info<Char>*, int, int) {}
 
 template <typename T> struct is_named_arg : std::false_type {};
+template <typename T> struct is_statically_named_arg : std::false_type {};
 
 template <typename T, typename Char>
 struct is_named_arg<named_arg<Char, T>> : std::true_type {};
@@ -2425,6 +2436,36 @@ class compile_parse_context
   using base::check_arg_id;
 };
 
+constexpr int invalid_arg_index = -1;
+
+#if FMT_USE_NONTYPE_TEMPLATE_PARAMETERS
+template <int N, typename T, typename... Args, typename Char>
+constexpr int get_arg_index_by_name(basic_string_view<Char> name) {
+  if constexpr (detail::is_statically_named_arg<T>()) {
+    if (name == T::name) return N;
+  }
+  if constexpr (sizeof...(Args) == 0) {
+    return invalid_arg_index;
+  } else {
+    return get_arg_index_by_name<N + 1, Args...>(name);
+  }
+}
+
+template <typename... Args, typename Char>
+constexpr int get_arg_index_by_name(basic_string_view<Char> name) {
+  if constexpr (sizeof...(Args) == 0) {
+    return invalid_arg_index;
+  } else {
+    return get_arg_index_by_name<0, Args...>(name);
+  }
+}
+#else
+template <typename... Args, typename Char>
+constexpr int get_arg_index_by_name(basic_string_view<Char>) {
+  return invalid_arg_index;
+}
+#endif
+
 template <typename Char, typename ErrorHandler, typename... Args>
 class format_string_checker {
  public:
@@ -2437,9 +2478,16 @@ class format_string_checker {
 
   FMT_CONSTEXPR int on_arg_id() { return context_.next_arg_id(); }
   FMT_CONSTEXPR int on_arg_id(int id) { return context_.check_arg_id(id), id; }
-  FMT_CONSTEXPR int on_arg_id(basic_string_view<Char>) {
+  FMT_CONSTEXPR int on_arg_id(basic_string_view<Char> id) {
-    on_error("compile-time checks don't support named arguments");
+#if FMT_USE_NONTYPE_TEMPLATE_PARAMETERS
+    auto index = get_arg_index_by_name<Args...>(id);
+    if (index == invalid_arg_index) on_error("named argument is not found");
+    return context_.check_arg_id(index), index;
+#else
+    (void)id;
+    on_error("compile-time checks for named arguments require C++20 support");
     return 0;
+#endif
   }
 
   FMT_CONSTEXPR void on_replacement_field(int, const Char*) {}

include/fmt/format.h

@@ -255,16 +255,6 @@ inline int ctzll(uint64_t x) {
 FMT_END_NAMESPACE
 #endif
 
-#ifndef FMT_USE_NONTYPE_TEMPLATE_PARAMETERS
-#  if defined(__cpp_nontype_template_args) &&                \
-      ((FMT_GCC_VERSION >= 903 && __cplusplus >= 201709L) || \
-       __cpp_nontype_template_args >= 201911L)
-#    define FMT_USE_NONTYPE_TEMPLATE_PARAMETERS 1
-#  else
-#    define FMT_USE_NONTYPE_TEMPLATE_PARAMETERS 0
-#  endif
-#endif
-
 FMT_BEGIN_NAMESPACE
 namespace detail {
 
@@ -3226,9 +3216,6 @@ template <typename Char> struct udl_formatter {
   }
 };
 
-template <typename T, typename = void>
-struct is_statically_named_arg : std::false_type {};
-
 #  if FMT_USE_NONTYPE_TEMPLATE_PARAMETERS
 template <typename T, typename Char, size_t N, fixed_string<Char, N> Str>
 struct statically_named_arg : view {

test/format-test.cc

@@ -1694,6 +1694,14 @@ TEST(format_test, compile_time_string) {
   EXPECT_EQ(L"42", fmt::format(FMT_STRING(L"{}"), 42));
   EXPECT_EQ("foo", fmt::format(FMT_STRING("{}"), string_like()));
 
+#if FMT_USE_NONTYPE_TEMPLATE_PARAMETERS
+  using namespace fmt::literals;
+  EXPECT_EQ("foobar", fmt::format(FMT_STRING("{foo}{bar}"), "bar"_a = "bar",
+                                  "foo"_a = "foo"));
+  EXPECT_EQ("", fmt::format(FMT_STRING("")));
+  EXPECT_EQ("", fmt::format(FMT_STRING(""), "arg"_a = 42));
+#endif
+
   (void)static_with_null;
   (void)static_with_null_wide;
   (void)static_no_null;
@@ -2339,8 +2347,15 @@ TEST(format_test, format_string_errors) {
 #  else
   fmt::print("warning: constexpr is broken in this version of MSVC\n");
 #  endif
-  EXPECT_ERROR("{foo", "compile-time checks don't support named arguments",
+#  if FMT_USE_NONTYPE_TEMPLATE_PARAMETERS
+  EXPECT_ERROR("{foo}", "named argument is not found", decltype("bar"_a = 42));
+  EXPECT_ERROR("{foo}", "named argument is not found",
+               decltype(fmt::arg("foo", 42)));
+#  else
+  EXPECT_ERROR("{foo}",
+               "compile-time checks for named arguments require C++20 support",
                int);
+#  endif
   EXPECT_ERROR_NOARGS("{10000000000}", "number is too big");
   EXPECT_ERROR_NOARGS("{0x}", "invalid format string");
   EXPECT_ERROR_NOARGS("{-}", "invalid format string");