<table border="1" cellspacing="0" cellpadding="8">
<tr>
<th>Issue</th>
<td>
<a href=https://github.com/llvm/llvm-project/issues/151328>151328</a>
</td>
</tr>
<tr>
<th>Summary</th>
<td>
[libc++] Regression with std::variant move construction
</td>
</tr>
<tr>
<th>Labels</th>
<td>
libc++
</td>
</tr>
<tr>
<th>Assignees</th>
<td>
</td>
</tr>
<tr>
<th>Reporter</th>
<td>
ldionne
</td>
</tr>
</table>
<pre>
This breaks some valid code. Here a partially reduced test case that depends on libc++ and Google Test: https://gcc.godbolt.org/z/dPznsz44M
```
#include <variant>
#include <vector>
#include <gmock/gmock.h>
template <typename T>
class Nullable {
public:
Nullable() : value_() {}
template <typename V>
Nullable(V value) : value_(value) {}
const T& value() const { return value_; }
private:
T value_;
};
template <typename L, typename R>
inline bool operator== (const Nullable<L>& lhs, const Nullable<R>& rhs) {
return lhs.value() == rhs.value();
}
void f() {
std::vector<std::variant<int, Nullable<int>>> v;
ASSERT_THAT(v, ::testing::ElementsAre(16));
}
```
It used to compile fine, but now it triggers a compilation error:
```
/opt/compiler-explorer/clang-trunk-20250722/bin/../include/c++/v1/variant:1186:37: fatal error: recursive template instantiation exceeded maximum depth of 1024
1186 | enable_if_t<!is_same_v<__remove_cvref_t<_Arg>, variant>, int> = 0,
| ^
/opt/compiler-explorer/clang-trunk-20250722/bin/../include/c++/v1/variant:1192:35: note: while substituting prior template arguments into non-type template parameter [with _Arg = testing::Matcher<const std::variant<int, Nullable<int>> &>]
1192 | _LIBCPP_HIDE_FROM_ABI constexpr variant(_Arg&& __arg) noexcept(is_nothrow_constructible_v<_Tp, _Arg>)
| ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
1193 | : __impl_(in_place_index<_Ip>, std::forward<_Arg>(__arg)) {}
| ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
/opt/compiler-explorer/clang-trunk-20250722/bin/../include/c++/v1/__type_traits/invoke.h:71:69: note: while substituting deduced template arguments into function template 'variant' [with _Arg = testing::Matcher<const std::variant<int, Nullable<int>> &>, $1 = (no value), $2 = (no value), $3 = (no value), _Tp = (no value), _Ip = (no value), $6 = (no value)]
71 | using __invoke_result_t _LIBCPP_NODEBUG = decltype(__builtin_invoke(std::declval<_Args>()...));
| ^
/opt/compiler-explorer/clang-trunk-20250722/bin/../include/c++/v1/__type_traits/invoke.h:387:1: note: in instantiation of template type alias '__invoke_result_t' requested here
387 | using invoke_result_t = __invoke_result_t<_Fn, _Args...>;
| ^
/opt/compiler-explorer/clang-trunk-20250722/bin/../include/c++/v1/variant:1140:1: note: in instantiation of template type alias 'invoke_result_t' requested here
1140 | using __best_match_t _LIBCPP_NODEBUG = typename invoke_result_t<_MakeOverloads<_Types...>, _Tp, _Tp>::type;
| ^
/opt/compiler-explorer/clang-trunk-20250722/bin/../include/c++/v1/variant:1189:45: note: in instantiation of template type alias '__best_match_t' requested here
1189 | class _Tp = __variant_detail::__best_match_t<_Arg, _Types...>,
| ^
/opt/compiler-explorer/clang-trunk-20250722/bin/../include/c++/v1/variant:1192:35: note: (skipping 2533 contexts in backtrace; use -ftemplate-backtrace-limit=0 to see all)
1192 | _LIBCPP_HIDE_FROM_ABI constexpr variant(_Arg&& __arg) noexcept(is_nothrow_constructible_v<_Tp, _Arg>)
| ^
/opt/compiler-explorer/libs/googletest/trunk/googlemock/include/gmock/gmock-matchers.h:356:12: note: in instantiation of function template specialization 'testing::internal::MatcherCastImpl<const std::vector<std::variant<int, Nullable<int>>> &, testing::internal::ElementsAreMatcher<std::tuple<int>>>::CastImpl<false>' requested here
356 | return CastImpl(polymorphic_matcher_or_value,
| ^
/opt/compiler-explorer/libs/googletest/trunk/googlemock/include/gmock/gmock-matchers.h:529:43: note: in instantiation of member function 'testing::internal::MatcherCastImpl<const std::vector<std::variant<int, Nullable<int>>> &, testing::internal::ElementsAreMatcher<std::tuple<int>>>::Cast' requested here
529 | return internal::MatcherCastImpl<T, M>::Cast(matcher);
| ^
/opt/compiler-explorer/libs/googletest/trunk/googlemock/include/gmock/gmock-matchers.h:536:10: note: in instantiation of function template specialization 'testing::MatcherCast<const std::vector<std::variant<int, Nullable<int>>> &, testing::internal::ElementsAreMatcher<std::tuple<int>>>' requested here
536 | return MatcherCast<T>(polymorphic_matcher_or_value);
| ^
/opt/compiler-explorer/libs/googletest/trunk/googlemock/include/gmock/gmock-matchers.h:1661:39: note: in instantiation of function template specialization 'testing::SafeMatcherCast<const std::vector<std::variant<int, Nullable<int>>> &, testing::internal::ElementsAreMatcher<std::tuple<int>>>' requested here
1661 | const Matcher<const T&> matcher = SafeMatcherCast<const T&>(matcher_);
| ^
<source>:26:3: note: in instantiation of function template specialization 'testing::internal::PredicateFormatterFromMatcher<testing::internal::ElementsAreMatcher<std::tuple<int>>>::operator()<std::vector<std::variant<int, Nullable<int>>>>' requested here
26 | ASSERT_THAT(v, ::testing::ElementsAre(16));
| ^
/opt/compiler-explorer/libs/googletest/trunk/googlemock/include/gmock/gmock-matchers.h:5736:3: note: expanded from macro 'ASSERT_THAT'
5736 | ASSERT_PRED_FORMAT1( \
| ^
/opt/compiler-explorer/libs/googletest/trunk/googletest/include/gtest/gtest_pred_impl.h:112:3: note: expanded from macro 'ASSERT_PRED_FORMAT1'
112 | GTEST_PRED_FORMAT1_(pred_format, v1, GTEST_FATAL_FAILURE_)
| ^
/opt/compiler-explorer/libs/googletest/trunk/googletest/include/gtest/gtest_pred_impl.h:100:28: note: expanded from macro 'GTEST_PRED_FORMAT1_'
100 | GTEST_ASSERT_(pred_format(#v1, v1), on_failure)
| ^
/opt/compiler-explorer/libs/googletest/trunk/googletest/include/gtest/gtest_pred_impl.h:79:52: note: expanded from macro 'GTEST_ASSERT_'
79 | if (const ::testing::AssertionResult gtest_ar = (expression)) \
| ^
1 error generated.
```
@philnik777 please take a look.
_Originally posted by @alexfh in https://github.com/llvm/llvm-project/issues/116709#issuecomment-3105095648_
</pre>
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