Implement nested formatter

include/fmt/core.h

@@ -2302,9 +2302,12 @@ FMT_CONSTEXPR FMT_INLINE auto parse_format_specs(
     dynamic_format_specs<Char>& specs;
     type arg_type;
 
-    FMT_CONSTEXPR auto operator()(pres type, int set) -> const Char* {
-      if (!in(arg_type, set)) throw_format_error("invalid format specifier");
-      specs.type = type;
+    FMT_CONSTEXPR auto operator()(pres pres_type, int set) -> const Char* {
+      if (!in(arg_type, set)) {
+        if (arg_type == type::none_type) return begin;
+        throw_format_error("invalid format specifier");
+      }
+      specs.type = pres_type;
       return begin + 1;
     }
   } parse_presentation_type{begin, specs, arg_type};
@@ -2321,6 +2324,7 @@ FMT_CONSTEXPR FMT_INLINE auto parse_format_specs(
     case '+':
     case '-':
     case ' ':
+      if (arg_type == type::none_type) return begin;
       enter_state(state::sign, in(arg_type, sint_set | float_set));
       switch (c) {
       case '+':
@@ -2336,14 +2340,17 @@ FMT_CONSTEXPR FMT_INLINE auto parse_format_specs(
       ++begin;
       break;
     case '#':
+      if (arg_type == type::none_type) return begin;
       enter_state(state::hash, is_arithmetic_type(arg_type));
       specs.alt = true;
       ++begin;
       break;
     case '0':
       enter_state(state::zero);
-      if (!is_arithmetic_type(arg_type))
+      if (!is_arithmetic_type(arg_type)) {
+        if (arg_type == type::none_type) return begin;
         throw_format_error("format specifier requires numeric argument");
+      }
       if (specs.align == align::none) {
         // Ignore 0 if align is specified for compatibility with std::format.
         specs.align = align::numeric;
@@ -2365,12 +2372,14 @@ FMT_CONSTEXPR FMT_INLINE auto parse_format_specs(
       begin = parse_dynamic_spec(begin, end, specs.width, specs.width_ref, ctx);
       break;
     case '.':
+      if (arg_type == type::none_type) return begin;
       enter_state(state::precision,
                   in(arg_type, float_set | string_set | cstring_set));
       begin = parse_precision(begin, end, specs.precision, specs.precision_ref,
                               ctx);
       break;
     case 'L':
+      if (arg_type == type::none_type) return begin;
       enter_state(state::locale, is_arithmetic_type(arg_type));
       specs.localized = true;
       ++begin;

include/fmt/format.h

@@ -4222,13 +4222,35 @@ struct formatter<nested_view<T>> {
 template <typename T>
 struct nested_formatter {
  private:
+  int width_;
+  detail::fill_t<char> fill_;
+  align_t align_ : 4;
   formatter<T> formatter_;
- 
+
  public:
   FMT_CONSTEXPR auto parse(format_parse_context& ctx) -> const char* {
+    auto specs = detail::dynamic_format_specs<char>();
+    auto it = parse_format_specs(
+      ctx.begin(), ctx.end(), specs, ctx, detail::type::none_type);
+    width_ = specs.width;
+    fill_ = specs.fill;
+    align_ = specs.align;
+    ctx.advance_to(it);
     return formatter_.parse(ctx);
   }
 
+  template <typename F>
+  auto write_padded(format_context& ctx, F write) const -> decltype(ctx.out()) {
+    if (width_ == 0) return write(ctx.out());
+    auto buf = memory_buffer();
+    write(std::back_inserter(buf));
+    auto specs = format_specs<>();
+    specs.width = width_;
+    specs.fill = fill_;
+    specs.align = align_;
+    return detail::write(ctx.out(), string_view(buf.data(), buf.size()), specs);
+  }
+
   auto nested(const T& value) const -> nested_view<T> {
     return nested_view<T>{&formatter_, &value};
   }

test/format-test.cc

@@ -1779,22 +1779,25 @@ TEST(format_test, group_digits_view) {
   EXPECT_EQ(fmt::format("{:8}", fmt::group_digits(1000)), "   1,000");
 }
 
+#ifdef __cpp_generic_lambdas
 struct point {
   double x, y;
 };
 
 FMT_BEGIN_NAMESPACE
-template <>
-struct formatter<point> : nested_formatter<double> {
+template <> struct formatter<point> : nested_formatter<double> {
   auto format(point p, format_context& ctx) const -> decltype(ctx.out()) {
-    return format_to(ctx.out(), "({}, {})", nested(p.x), nested(p.y));
+    return write_padded(ctx, [this, p](auto out) -> decltype(out) {
+      return format_to(out, "({}, {})", nested(p.x), nested(p.y));
+    });
   }
 };
 FMT_END_NAMESPACE
 
 TEST(format_test, nested_formatter) {
-  EXPECT_EQ(fmt::format("{:.2f}", point{1, 2}), "(1.00, 2.00)");
+  EXPECT_EQ(fmt::format("{:>16.2f}", point{1, 2}), "    (1.00, 2.00)");
 }
+#endif  // __cpp_generic_lambdas
 
 enum test_enum { foo, bar };
 auto format_as(test_enum e) -> int { return e; }