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.

.travis.yml

@@ -260,7 +260,7 @@ matrix:
       addons:
           apt:
               sources: *all_sources
-              packages: ['clang-6.0', 'libstdc++-7-dev']
+              packages: ['clang-6.0', 'libstdc++-8-dev']
       env: COMPILER='clang++-6.0' CPP17=1
 
     - os: linux
@@ -268,7 +268,7 @@ matrix:
       addons:
           apt:
               sources: *all_sources
-              packages: ['clang-6.0', 'libstdc++-7-dev']
+              packages: ['clang-6.0', 'libstdc++-8-dev']
       env: COMPILER='clang++-6.0' CPP17=1 EXAMPLES=1 COVERAGE=1 EXTRAS=1
 
 install:

docs/configuration.md

@@ -128,6 +128,7 @@ Catch's selection, by defining either `CATCH_CONFIG_CPP11_TO_STRING` or
 
     CATCH_CONFIG_CPP17_UNCAUGHT_EXCEPTIONS  // Use std::uncaught_exceptions instead of std::uncaught_exception
     CATCH_CONFIG_CPP17_STRING_VIEW          // Provide StringMaker specialization for std::string_view
+    CATCH_CONFIG_CPP17_VARIANT              // Override C++17 detection for CATCH_CONFIG_ENABLE_VARIANT_STRINGMAKER
 
 Catch contains basic compiler/standard detection and attempts to use
 some C++17 features whenever appropriate. This automatic detection
@@ -202,6 +203,7 @@ By default, Catch does not stringify some types from the standard library. This
     CATCH_CONFIG_ENABLE_PAIR_STRINGMAKER    // Provide StringMaker specialization for std::pair
     CATCH_CONFIG_ENABLE_TUPLE_STRINGMAKER   // Provide StringMaker specialization for std::tuple
     CATCH_CONFIG_ENABLE_CHRONO_STRINGMAKER  // Provide StringMaker specialization for std::chrono::duration, std::chrono::timepoint
+    CATCH_CONFIG_ENABLE_VARIANT_STRINGMAKER // Provide StringMaker specialization for std::variant, std::monostate (on C++17)
     CATCH_CONFIG_ENABLE_ALL_STRINGMAKERS    // Defines all of the above
 
 

include/internal/catch_compiler_capabilities.h

@@ -163,6 +163,23 @@
 #endif
 #endif
 
+////////////////////////////////////////////////////////////////////////////////
+// Check if variant is available and usable
+#if defined(__has_include)
+#  if __has_include(<variant>) && defined(CATCH_CPP17_OR_GREATER)
+#    if defined(__clang__) && (__clang_major__ < 8)
+       // work around clang bug with libstdc++ https://bugs.llvm.org/show_bug.cgi?id=31852
+       // fix should be in clang 8, workaround in libstdc++ 8.2
+#      include <ciso646>
+#      if defined(__GLIBCXX__) && defined(_GLIBCXX_RELEASE) && (_GLIBCXX_RELEASE < 9)
+#        define CATCH_CONFIG_NO_CPP17_VARIANT
+#     else
+#        define CATCH_INTERNAL_CONFIG_CPP17_VARIANT
+#      endif // defined(__GLIBCXX__) && defined(_GLIBCXX_RELEASE) && (_GLIBCXX_RELEASE < 9)
+#    endif // defined(__clang__) && (__clang_major__ < 8)
+#  endif // __has_include(<variant>) && defined(CATCH_CPP17_OR_GREATER)
+#endif // __has_include
+
 
 #if defined(CATCH_INTERNAL_CONFIG_COUNTER) && !defined(CATCH_CONFIG_NO_COUNTER) && !defined(CATCH_CONFIG_COUNTER)
 #   define CATCH_CONFIG_COUNTER
@@ -191,6 +208,10 @@
 #  define CATCH_CONFIG_CPP17_STRING_VIEW
 #endif
 
+#if defined(CATCH_INTERNAL_CONFIG_CPP17_VARIANT) && !defined(CATCH_CONFIG_NO_CPP17_VARIANT) && !defined(CATCH_CONFIG_CPP17_VARIANT)
+#  define CATCH_CONFIG_CPP17_VARIANT
+#endif
+
 #if defined(CATCH_CONFIG_EXPERIMENTAL_REDIRECT)
 #  define CATCH_INTERNAL_CONFIG_NEW_CAPTURE
 #endif

include/internal/catch_tostring.h

@@ -354,6 +354,7 @@ namespace Catch {
 #if defined(CATCH_CONFIG_ENABLE_ALL_STRINGMAKERS)
 #  define CATCH_CONFIG_ENABLE_PAIR_STRINGMAKER
 #  define CATCH_CONFIG_ENABLE_TUPLE_STRINGMAKER
+#  define CATCH_CONFIG_ENABLE_VARIANT_STRINGMAKER
 #  define CATCH_CONFIG_ENABLE_CHRONO_STRINGMAKER
 #endif
 
@@ -418,6 +419,34 @@ namespace Catch {
 }
 #endif // CATCH_CONFIG_ENABLE_TUPLE_STRINGMAKER
 
+#if defined(CATCH_CONFIG_ENABLE_VARIANT_STRINGMAKER) && defined(CATCH_CONFIG_CPP17_VARIANT)
+#include <variant>
+namespace Catch {
+    template<>
+    struct StringMaker<std::monostate> {
+        static std::string convert(const std::monostate&) {
+            return "{ }";
+        }
+    };
+
+    template<typename... Elements>
+    struct StringMaker<std::variant<Elements...>> {
+        static std::string convert(const std::variant<Elements...>& variant) {
+            if (variant.valueless_by_exception()) {
+                return "{valueless variant}";
+            } else {
+                return std::visit(
+                    [](const auto& value) {
+                        return ::Catch::Detail::stringify(value);
+                    },
+                    variant
+                );
+            }
+        }
+    };
+}
+#endif // CATCH_CONFIG_ENABLE_VARIANT_STRINGMAKER
+
 namespace Catch {
     struct not_this_one {}; // Tag type for detecting which begin/ end are being selected
 

projects/CMakeLists.txt

@@ -38,6 +38,7 @@ set(TEST_SOURCES
         ${SELF_TEST_DIR}/UsageTests/ToStringGeneral.tests.cpp
         ${SELF_TEST_DIR}/UsageTests/ToStringPair.tests.cpp
         ${SELF_TEST_DIR}/UsageTests/ToStringTuple.tests.cpp
+        ${SELF_TEST_DIR}/UsageTests/ToStringVariant.tests.cpp
         ${SELF_TEST_DIR}/UsageTests/ToStringVector.tests.cpp
         ${SELF_TEST_DIR}/UsageTests/ToStringWhich.tests.cpp
         ${SELF_TEST_DIR}/UsageTests/Tricky.tests.cpp

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