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Exclude std::optional from generic range StringMaker (C++26 P3168 fix) #3093

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Exclude std::optional from generic range StringMaker (C++26 P3168 fix) #3093

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18 changes: 17 additions & 1 deletion src/catch2/catch_tostring.hpp
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Original file line number Diff line number Diff line change
Expand Up @@ -25,6 +25,10 @@
#include <string_view>
#endif

#ifdef CATCH_CONFIG_CPP17_OPTIONAL
#include <optional>
#endif

#ifdef _MSC_VER
#pragma warning(push)
#pragma warning(disable:4180) // We attempt to stream a function (address) by const&, which MSVC complains about but is harmless
Expand Down Expand Up @@ -481,6 +485,16 @@ namespace Catch {

template <typename T>
struct is_range_impl<T, void_t<decltype(begin(std::declval<T>()))>> : std::true_type {};

// Used to exclude std::optional from the generic range StringMaker so it
// does not collide with the dedicated optional StringMaker (or a
// user-provided one). std::optional models std::ranges::range since
// C++26 (P3168), so is_range<std::optional<T>> is true and both partial
// specializations would otherwise match StringMaker<std::optional<T>>.
template <typename T> struct is_optional : std::false_type {};
#if defined(CATCH_CONFIG_CPP17_OPTIONAL)
template <typename T> struct is_optional<std::optional<T>> : std::true_type {};
#endif
} // namespace Detail

template <typename T>
Expand Down Expand Up @@ -516,7 +530,9 @@ namespace Catch {
}

template<typename R>
struct StringMaker<R, std::enable_if_t<is_range<R>::value && !::Catch::Detail::IsStreamInsertable_v<R>>> {
struct StringMaker<R, std::enable_if_t<is_range<R>::value
&& !::Catch::Detail::IsStreamInsertable_v<R>
&& !::Catch::Detail::is_optional<R>::value>> {
static std::string convert( R const& range ) {
return rangeToString( range );
}
Expand Down
15 changes: 15 additions & 0 deletions tests/SelfTest/UsageTests/ToStringOptional.tests.cpp
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Expand Up @@ -32,4 +32,19 @@ TEST_CASE( "std::nullopt -> toString", "[toString][optional][approvals]" ) {
REQUIRE( "{ }" == ::Catch::Detail::stringify( std::nullopt ) );
}

namespace {
struct NonStreamable { int v; };
}

// Regression: under C++26, std::optional models std::ranges::range (P3168), so
// without the is_optional carve-out in the range StringMaker the dedicated
// optional StringMaker collides with the range one and instantiation becomes
// ambiguous. This test just needs to compile.
TEST_CASE( "std::optional<NonStreamable> -> toString does not collide with range StringMaker",
"[toString][optional][approvals]" ) {
using type = std::optional<NonStreamable>;
REQUIRE( "{ }" == ::Catch::Detail::stringify( type{} ) );
REQUIRE( "{?}" == ::Catch::Detail::stringify( type{ NonStreamable{ 42 } } ) );
}

#endif // CATCH_INTERNAL_CONFIG_CPP17_OPTIONAL