Add support for 'std::variant' in C++17.

For C++17, if all the alternatives of a variant are formattable
the variant is now also formattable. In addition 'std::monostate'
is now formattable.

The value of a variant is enclosed in '<' and '>', and the monostate
is formatted as ' '.

doc/api.rst

@@ -12,6 +12,7 @@ The {fmt} library API consists of the following parts:
   formatting functions and locale support
 * :ref:`fmt/ranges.h <ranges-api>`: formatting of ranges and tuples
 * :ref:`fmt/chrono.h <chrono-api>`: date and time formatting
+* :ref:`fmt/variant.h <variant-api>`: formatting of variants
 * :ref:`fmt/compile.h <compile-api>`: format string compilation
 * :ref:`fmt/color.h <color-api>`: terminal color and text style
 * :ref:`fmt/os.h <os-api>`: system APIs
@@ -181,8 +182,9 @@ Formatting User-defined Types
 
 The {fmt} library provides formatters for many standard C++ types.
 See :ref:`fmt/ranges.h <ranges-api>` for ranges and tuples including standard
-containers such as ``std::vector`` and :ref:`fmt/chrono.h <chrono-api>` for date
+containers such as ``std::vector``, :ref:`fmt/chrono.h <chrono-api>` for date
-and time formatting.
+and time formatting and :ref:`fmt/variant.h <variant-api>` for variant 
+formatting.
 
 To make a user-defined type formattable, specialize the ``formatter<T>`` struct
 template and implement ``parse`` and ``format`` methods::
@@ -447,6 +449,31 @@ The format syntax is described in :ref:`chrono-specs`.
 
 .. doxygenfunction:: gmtime(std::time_t time)
 
+.. _variant-api:
+
+Variant Formatting
+==================
+
+``fmt/variant.h`` provides formatters for
+
+* `std::monostate <https://en.cppreference.com/w/cpp/utility/variant/monostate>`_
+* `std::variant <https://en.cppreference.com/w/cpp/utility/variant/variant>`_
+
+**Example**::
+
+  #include <fmt/variant.h>
+
+  std::variant<char, float> v0{'x'};
+  // Prints "<'x'>"
+  fmt::print("{}", v0);
+
+  std::variant<std::monostate, char> v1{};
+  // Prints "< >"
+
+.. note::
+
+   Variant support is only available for C++17 and up.
+
 .. _compile-api:
 
 Format string compilation

doc/syntax.rst

@@ -346,7 +346,7 @@ points are:
 |         | command ``%OS`` produces the locale's alternative representation.  |
 +---------+--------------------------------------------------------------------+
 
-Specifiers that have a calendaric component such as `'d'` (the day of month)
+Specifiers that have a calendaric component such as ``'d'`` (the day of month)
 are valid only for ``std::tm`` and not durations or time points.
 
 .. range-specs:

include/fmt/variant.h

@@ -0,0 +1,107 @@
+// Formatting library for C++ - experimental range support
+//
+// {fmt} support for variant interface.
+
+#ifndef FMT_VARIANT_H_
+#define FMT_VARIANT_H_
+
+#include <type_traits>
+
+#include "format.h"
+#include "ranges.h"
+
+#define FMT_HAS_VARIANT FMT_CPLUSPLUS >= 201703L
+#if FMT_HAS_VARIANT
+
+#include <variant>
+
+FMT_BEGIN_NAMESPACE
+
+template <typename Char> struct formatter<std::monostate, Char> {
+  template <typename ParseContext>
+  FMT_CONSTEXPR auto parse(ParseContext& ctx) -> decltype(ctx.begin()) {
+    return ctx.begin();
+  }
+
+  template <typename FormatContext = format_context>
+  auto format(const std::monostate&, FormatContext& ctx) const
+      -> decltype(ctx.out()) {
+    auto out = ctx.out();
+    *out++ = ' ';
+    return out;
+  }
+};
+
+namespace detail {
+
+template <typename T>
+using variant_index_sequence = make_index_sequence<std::variant_size<T>::value>;
+
+// variant_size and variant_alternative check.
+template <typename T> class is_variant_like_ {
+  template <typename U>
+  static auto check(U* p) -> decltype(std::variant_size<U>::value, int());
+  template <typename> static void check(...);
+
+ public:
+  static constexpr const bool value =
+      !std::is_void<decltype(check<T>(nullptr))>::value;
+};
+
+// formattable element check
+template <typename T, typename C, bool = is_variant_like_<T>::value>
+class is_variant_formattable_ {
+ public:
+  static constexpr const bool value = false;
+};
+template <typename T, typename C> class is_variant_formattable_<T, C, true> {
+  template <std::size_t... I>
+  static std::integral_constant<
+      bool,
+      (is_formattable<std::variant_alternative_t<I, T>, C>::value && ...)>
+      check(index_sequence<I...>);
+
+ public:
+  static constexpr const bool value =
+      decltype(check(variant_index_sequence<T>{}))::value;
+};
+
+}  // namespace detail
+
+template <typename T> struct is_variant_like {
+  static constexpr const bool value = detail::is_variant_like_<T>::value;
+};
+
+template <typename T, typename C> struct is_variant_formattable {
+  static constexpr const bool value =
+      detail::is_variant_formattable_<T, C>::value;
+};
+
+template <typename VariantT, typename Char>
+struct formatter<
+    VariantT, Char,
+    enable_if_t<fmt::is_variant_like<VariantT>::value &&
+                fmt::is_variant_formattable<VariantT, Char>::value>> {
+  template <typename ParseContext>
+  FMT_CONSTEXPR auto parse(ParseContext& ctx) -> decltype(ctx.begin()) {
+    return ctx.begin();
+  }
+
+  template <typename FormatContext = format_context>
+  auto format(const VariantT& value, FormatContext& ctx) const
+      -> decltype(ctx.out()) {
+    auto out = ctx.out();
+    *out++ = '<';
+    std::visit(
+        [&](const auto& v) { out = detail::write_range_entry<Char>(out, v); },
+        value);
+    *out++ = '>';
+    return out;
+  }
+};
+
+FMT_END_NAMESPACE
+
+#endif  // FMT_VARIANT_H_
+
+#endif  // FMT_HAS_VARIANT

test/CMakeLists.txt

@@ -76,6 +76,7 @@ if (MSVC)
 endif()
 add_fmt_test(printf-test)
 add_fmt_test(ranges-test ranges-odr-test.cc)
+add_fmt_test(variant-test)
 add_fmt_test(scan-test)
 add_fmt_test(std-test)
 add_fmt_test(unicode-test HEADER_ONLY)

test/variant-test.cc

@@ -0,0 +1,43 @@
+// Formatting library for C++ - experimental variant API
+//
+// {fmt} support for variant interface.
+
+#include "fmt/variant.h"
+
+#include <string>
+
+#include "gtest/gtest.h"
+
+#if FMT_HAS_VARIANT
+
+TEST(variant_test, format_monostate) {
+  EXPECT_EQ(fmt::format("{}", std::monostate{}), " ");
+}
+TEST(variant_test, format_variant) {
+  using V0 = std::variant<int, float, std::string, char>;
+  V0 v0(42);
+  V0 v1(1.5f);
+  V0 v2("hello");
+  V0 v3('i');
+  EXPECT_EQ(fmt::format("{}", v0), "<42>");
+  EXPECT_EQ(fmt::format("{}", v1), "<1.5>");
+  EXPECT_EQ(fmt::format("{}", v2), "<\"hello\">");
+  EXPECT_EQ(fmt::format("{}", v3), "<'i'>");
+
+  struct unformattable{};
+  EXPECT_FALSE((fmt::is_formattable<unformattable>::value));
+  EXPECT_FALSE((fmt::is_formattable<std::variant<unformattable>>::value));
+  EXPECT_FALSE((fmt::is_formattable<std::variant<unformattable,int>>::value));
+  EXPECT_FALSE((fmt::is_formattable<std::variant<int,unformattable>>::value));
+  EXPECT_FALSE((fmt::is_formattable<std::variant<unformattable,unformattable>>::value));
+  EXPECT_TRUE((fmt::is_formattable<std::variant<int,float>>::value));
+
+  using V1 = std::variant<std::monostate, std::string, std::string>;
+  V1 v4{};
+  V1 v5{std::in_place_index<1>,"yes, this is variant"};
+
+  EXPECT_EQ(fmt::format("{}", v4), "< >");
+  EXPECT_EQ(fmt::format("{}", v5), "<\"yes, this is variant\">");
+}
+
+#endif // FMT_HAS_VARIANT