Add std::formatter support for variant2 and corresponding tests

Author: bigerl

include/dice/template-library/variant2.hpp

@@ -5,6 +5,7 @@
 #include <compare>
 #include <cstdint>
 #include <exception>
+#include <format>
 #include <functional>
 #include <type_traits>
 #include <utility>
@@ -903,4 +904,28 @@ namespace dice::template_library {
 
 } // namespace dice::template_library
 
+namespace std {
+    template<typename T, typename U, typename CharT>
+    struct formatter<::dice::template_library::variant2<T, U>, CharT> {
+        template<typename Ctx>
+        constexpr auto parse(Ctx &parse_ctx) {
+            return parse_ctx.begin();
+        }
+
+        template<typename Ctx>
+        auto format(::dice::template_library::variant2<T, U> const &var, Ctx &format_ctx) const {
+            if (var.valueless_by_exception()) {
+                return std::format_to(format_ctx.out(), "var2<valueless-by-exception>");
+            }
+            return ::dice::template_library::visit([&format_ctx]<typename X>(X const &value) {
+                if constexpr (std::formattable<X, CharT>) {
+                    return std::format_to(format_ctx.out(), "var2<{}>", value);
+                } else {
+                    return std::format_to(format_ctx.out(), "var2<non-formattable>");
+                }
+            }, var);
+        }
+    };
+} // namespace std
+
 #endif // DICE_TEMPLATELIBRARY_VARIANT2_HPP

tests/tests_variant2.cpp

@@ -289,4 +289,19 @@ TEST_SUITE("variant2") {
 	    static_assert(!std::is_trivially_destructible_v<var2>);
 	    static_assert(!std::is_trivially_copyable_v<var2>);
 	}
+
+    TEST_CASE("formatting") {
+	    struct non_formattable {
+	    };
+
+	    CHECK(std::format("{}", variant2<int, double>{42}) == "var2<42>");
+	    CHECK(std::format("{}", variant2<int, double>{1.5}) == "var2<1.5>");
+	    CHECK(std::format("{}", variant2<int, non_formattable>{5}) == "var2<5>");
+	    CHECK(std::format("{}", variant2<int, non_formattable>{non_formattable{}}) == "var2<non-formattable>");
+
+	    variant2<int, make_valueless> valueless{};
+	    try { valueless = make_valueless{std::nothrow}; } catch (...) {
+	    }
+	    CHECK(std::format("{}", valueless) == "var2<valueless-by-exception>");
+    }
 }