Add StringMaker for std::variant, std::monostate (#1380)

The StringMaker is off by default and can be enabled by a new macro `CATCH_CONFIG_ENABLE_VARIANT_STRINGMAKER`, to avoid increasing the footprint of stringification machinery by default.

projects/SelfTest/UsageTests/ToStringVariant.tests.cpp

@@ -0,0 +1,84 @@
+#define CATCH_CONFIG_ENABLE_VARIANT_STRINGMAKER
+#include "catch.hpp"
+
+#if defined(CATCH_CONFIG_CPP17_VARIANT)
+
+#include <string>
+#include <variant>
+
+TEST_CASE( "variant<std::monostate>", "[toString][variant][approvals]")
+{
+    using type = std::variant<std::monostate>;
+    CHECK( "{ }" == ::Catch::Detail::stringify(type{}) );
+    type value {};
+    CHECK( "{ }" == ::Catch::Detail::stringify(value) );
+    CHECK( "{ }" == ::Catch::Detail::stringify(std::get<0>(value)) );
+}
+
+TEST_CASE( "variant<int>", "[toString][variant][approvals]")
+{
+    using type = std::variant<int>;
+    CHECK( "0" == ::Catch::Detail::stringify(type{0}) );
+}
+
+TEST_CASE( "variant<float, int>", "[toString][variant][approvals]")
+{
+    using type = std::variant<float, int>;
+    CHECK( "0.5f" == ::Catch::Detail::stringify(type{0.5f}) );
+    CHECK( "0" == ::Catch::Detail::stringify(type{0}) );
+
+    SECTION("valueless by exception") {
+        struct sample {
+            operator int() const { throw 42; }
+        };
+
+        type value{1.5f};
+        REQUIRE_THROWS_AS( value.emplace<int>(sample{}), int );
+        REQUIRE( value.valueless_by_exception() );
+        CHECK( "{valueless variant}" == ::Catch::Detail::stringify(value) );
+    }
+}
+
+TEST_CASE( "variant<string, int>", "[toString][variant][approvals]")
+{
+    using type = std::variant<std::string, int>;
+    CHECK( "\"foo\"" == ::Catch::Detail::stringify(type{"foo"}) );
+    CHECK( "0" == ::Catch::Detail::stringify(type{0}) );
+}
+
+TEST_CASE( "variant<variant<float, int>, string>", "[toString][variant][approvals]")
+{
+    using inner = std::variant<float, int>;
+    using type = std::variant<inner, std::string>;
+    CHECK( "0.5f" == ::Catch::Detail::stringify(type{0.5f}) );
+    CHECK( "0" == ::Catch::Detail::stringify(type{0}) );
+    CHECK( "\"foo\"" == ::Catch::Detail::stringify(type{"foo"}) );
+
+    struct sample {
+        operator int() const { throw 42; }
+    };
+
+    SECTION("valueless nested variant") {
+        type value = inner{0.5f};
+        REQUIRE( std::holds_alternative<inner>(value) );
+        REQUIRE( std::holds_alternative<float>(std::get<inner>(value)) );
+
+        REQUIRE_THROWS_AS( std::get<0>(value).emplace<int>(sample{}), int );
+
+        // outer variant is still valid and contains inner
+        REQUIRE( std::holds_alternative<inner>(value) );
+        // inner variant is valueless
+        REQUIRE( std::get<inner>(value).valueless_by_exception() );
+        CHECK( "{valueless variant}" == ::Catch::Detail::stringify(value) );
+    }
+}
+
+TEST_CASE( "variant<nullptr,int,const char *>", "[toString][variant][approvals]" )
+{
+    using type = std::variant<std::nullptr_t,int,const char *>;
+    CHECK( "nullptr" == ::Catch::Detail::stringify(type{nullptr}) );
+    CHECK( "42" == ::Catch::Detail::stringify(type{42}) );
+    CHECK( "\"Catch me\"" == ::Catch::Detail::stringify(type{"Catch me"}) );
+}
+
+#endif // CATCH_INTERNAL_CONFIG_CPP17_VARIANT