Add support for 'std::variant' in C++17 (#2941)

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.

Moves implementation into 'std.h', and tests into 'std-test.cc'.

Avoid fold-expression since MSVC was crashing.

Add section for 'fmt/std.h' in API-docs.

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/std.h <std-api>`: formatters for standard library types
 * :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
@@ -184,8 +185,9 @@ 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/std.h <std-api>` for filesystem and variant 
+formatting.
 
 To make a user-defined type formattable, specialize the ``formatter<T>`` struct
 template and implement ``parse`` and ``format`` methods::
@@ -445,6 +447,35 @@ The format syntax is described in :ref:`chrono-specs`.
 
 .. doxygenfunction:: gmtime(std::time_t time)
 
+.. _std-api:
+
+Standard Library Types Formatting
+=================================
+
+``fmt/std.h`` provides formatters for:
+
+* `std::filesystem::path <std::filesystem::path>`_
+* `std::thread::id <https://en.cppreference.com/w/cpp/thread/thread/id>`_
+* `std::monostate <https://en.cppreference.com/w/cpp/utility/variant/monostate>`_
+* `std::variant <https://en.cppreference.com/w/cpp/utility/variant/variant>`_
+
+Formatting Variants
+-------------------
+
+A ``std::variant`` is only formattable if every variant alternative is formattable, and requires the
+``__cpp_lib_variant`` `library feature <https://en.cppreference.com/w/cpp/feature_test>`_.
+  
+**Example**::
+
+  #include <fmt/std.h>
+
+  std::variant<char, float> v0{'x'};
+  // Prints "variant('x')"
+  fmt::print("{}", v0);
+
+  std::variant<std::monostate, char> v1;
+  // Prints "variant(monostate)"
+
 .. _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/std.h

@@ -9,6 +9,8 @@
 #define FMT_STD_H_
 
 #include <thread>
+#include <type_traits>
+#include <utility>
 
 #include "ostream.h"
 
@@ -68,4 +70,97 @@ template <typename Char>
 struct formatter<std::thread::id, Char> : basic_ostream_formatter<Char> {};
 FMT_END_NAMESPACE
 
+#ifdef __cpp_lib_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>
+  auto format(const std::monostate&, FormatContext& ctx) const
+      -> decltype(ctx.out()) {
+    auto out = ctx.out();
+    out = detail::write<Char>(out, "monostate");
+    return out;
+  }
+};
+
+namespace detail {
+
+template <typename T>
+using variant_index_sequence =
+    std::make_index_sequence<std::variant_size<T>::value>;
+
+// variant_size and variant_alternative check.
+template <typename T, typename U = void>
+struct is_variant_like_ : std::false_type {};
+template <typename T>
+struct is_variant_like_<T, std::void_t<decltype(std::variant_size<T>::value)>>
+    : std::true_type {};
+
+// formattable element check
+template <typename T, typename C> class is_variant_formattable_ {
+  template <std::size_t... I>
+  static std::conjunction<
+      is_formattable<std::variant_alternative_t<I, T>, C>...>
+      check(std::index_sequence<I...>);
+
+ public:
+  static constexpr const bool value =
+      decltype(check(variant_index_sequence<T>{}))::value;
+};
+
+template <typename Char, typename OutputIt, typename T>
+auto write_variant_alternative(OutputIt out, const T& v) -> OutputIt {
+  if constexpr (is_string<T>::value)
+    return write_escaped_string<Char>(out, detail::to_string_view(v));
+  else if constexpr (std::is_same_v<T, Char>)
+    return write_escaped_char(out, v);
+  else
+    return write<Char>(out, v);
+}
+
+}  // 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 Variant, typename Char>
+struct formatter<
+    Variant, Char,
+    std::enable_if_t<std::conjunction_v<
+        is_variant_like<Variant>, is_variant_formattable<Variant, Char>>>> {
+  template <typename ParseContext>
+  FMT_CONSTEXPR auto parse(ParseContext& ctx) -> decltype(ctx.begin()) {
+    return ctx.begin();
+  }
+
+  template <typename FormatContext>
+  auto format(const Variant& value, FormatContext& ctx) const
+      -> decltype(ctx.out()) {
+    auto out = ctx.out();
+
+    out = detail::write<Char>(out, "variant(");
+    std::visit(
+        [&](const auto& v) {
+          out = detail::write_variant_alternative<Char>(out, v);
+        },
+        value);
+    *out++ = ')';
+    return out;
+  }
+};
+FMT_END_NAMESPACE
+#endif
+
 #endif  // FMT_STD_H_

test/std-test.cc

@@ -7,6 +7,8 @@
 
 #include "fmt/std.h"
 
+#include <string>
+
 #include "gtest/gtest.h"
 
 TEST(std_test, path) {
@@ -32,3 +34,34 @@ TEST(std_test, path) {
 TEST(std_test, thread_id) {
   EXPECT_FALSE(fmt::format("{}", std::this_thread::get_id()).empty());
 }
+
+TEST(std_test, variant) {
+#ifdef __cpp_lib_variant
+  EXPECT_EQ(fmt::format("{}", std::monostate{}), "monostate");
+  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), "variant(42)");
+  EXPECT_EQ(fmt::format("{}", v1), "variant(1.5)");
+  EXPECT_EQ(fmt::format("{}", v2), "variant(\"hello\")");
+  EXPECT_EQ(fmt::format("{}", v3), "variant('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), "variant(monostate)");
+  EXPECT_EQ(fmt::format("{}", v5), "variant(\"yes, this is variant\")");
+#endif
+}