From c882790a2e0134e7943fc70e42a99d210f5f2334 Mon Sep 17 00:00:00 2001
From: Victor Zverovich <viz@fb.com>
Date: Sat, 18 Dec 2021 07:35:40 -0800
Subject: [PATCH] Add a set formatter

---
 include/fmt/ranges.h | 55 ++++++++++++++++++++++++--------------------
 test/ranges-test.cc  |  5 ++++
 2 files changed, 35 insertions(+), 25 deletions(-)

diff --git a/include/fmt/ranges.h b/include/fmt/ranges.h
index 3068649f..71474f32 100644
--- a/include/fmt/ranges.h
+++ b/include/fmt/ranges.h
@@ -19,21 +19,6 @@
 
 FMT_BEGIN_NAMESPACE
 
-template <typename Char, typename Enable = void> struct formatting_range {
-#ifdef FMT_DEPRECATED_BRACED_RANGES
-  Char prefix = '{';
-  Char postfix = '}';
-#else
-  Char prefix = '[';
-  Char postfix = ']';
-#endif
-
-  template <typename ParseContext>
-  FMT_CONSTEXPR auto parse(ParseContext& ctx) -> decltype(ctx.begin()) {
-    return ctx.begin();
-  }
-};
-
 template <typename Char, typename Enable = void> struct formatting_tuple {
   Char prefix = '(';
   Char postfix = ')';
@@ -87,7 +72,7 @@ template <typename Char>
 struct is_std_string_like<fmt::basic_string_view<Char>> : std::true_type {};
 
 template <typename T> class is_map {
-  template <typename U> static auto check(U*) -> typename U::key_type;
+  template <typename U> static auto check(U*) -> typename U::mapped_type;
   template <typename> static void check(...);
 
  public:
@@ -99,6 +84,19 @@ template <typename T> class is_map {
 #endif
 };
 
+template <typename T> class is_set {
+  template <typename U> static auto check(U*) -> typename U::key_type;
+  template <typename> static void check(...);
+
+ public:
+#ifdef FMT_FORMAT_SET_AS_LIST
+  static FMT_CONSTEXPR_DECL const bool value = false;
+#else
+  static FMT_CONSTEXPR_DECL const bool value =
+      !std::is_void<decltype(check<T>(nullptr))>::value && !is_map<T>::value;
+#endif
+};
+
 template <typename... Ts> struct conditional_helper {};
 
 template <typename T, typename _ = void> struct is_range_ : std::false_type {};
@@ -602,11 +600,9 @@ struct formatter<
             detail::has_fallback_formatter<detail::value_type<T>, Char>::value)
 #endif
         >> {
-  formatting_range<Char> formatting;
-
   template <typename ParseContext>
   FMT_CONSTEXPR auto parse(ParseContext& ctx) -> decltype(ctx.begin()) {
-    return formatting.parse(ctx);
+    return ctx.begin();
   }
 
   template <
@@ -614,17 +610,26 @@ struct formatter<
       FMT_ENABLE_IF(
           std::is_same<U, conditional_t<detail::has_const_begin_end<T>::value,
                                         const T, T>>::value)>
-  auto format(U& values, FormatContext& ctx) -> decltype(ctx.out()) {
-    auto out = detail::copy(formatting.prefix, ctx.out());
-    size_t i = 0;
-    auto it = std::begin(values);
-    auto end = std::end(values);
+  auto format(U& range, FormatContext& ctx) -> decltype(ctx.out()) {
+#ifdef FMT_DEPRECATED_BRACED_RANGES
+    Char prefix = '{';
+    Char postfix = '}';
+#else
+    Char prefix = detail::is_set<T>::value ? '{' : '[';
+    Char postfix = detail::is_set<T>::value ? '}' : ']';
+#endif
+    auto out = ctx.out();
+    *out++ = prefix;
+    int i = 0;
+    auto it = std::begin(range);
+    auto end = std::end(range);
     for (; it != end; ++it) {
       if (i > 0) out = detail::write_delimiter(out);
       out = detail::write_range_entry<Char>(out, *it);
       ++i;
     }
-    return detail::copy(formatting.postfix, out);
+    *out++ = postfix;
+    return out;
   }
 };
 
diff --git a/test/ranges-test.cc b/test/ranges-test.cc
index 4481e2b4..86dd5520 100644
--- a/test/ranges-test.cc
+++ b/test/ranges-test.cc
@@ -58,6 +58,11 @@ TEST(ranges_test, format_map) {
   EXPECT_EQ(fmt::format("{}", m), "{\"one\": 1, \"two\": 2}");
 }
 
+TEST(ranges_test, format_set) {
+  EXPECT_EQ(fmt::format("{}", std::set<std::string>{"one", "two"}),
+            "{\"one\", \"two\"}");
+}
+
 TEST(ranges_test, format_pair) {
   auto p = std::pair<int, float>(42, 1.5f);
   EXPECT_EQ(fmt::format("{}", p), "(42, 1.5)");