alexfh wrote:
The reduced test case: https://gcc.godbolt.org/z/KhebWaGsc
```
#include <variant>
#include <vector>
namespace testing {
template <typename T> struct Matcher {
Matcher(int);
Matcher(T);
};
template <typename T, typename M>
Matcher<T> SafeMatcherCast(M polymorphic_matcher_or_value) {
return polymorphic_matcher_or_value;
}
namespace internal {
template <typename MatcherTuple> struct ElementsAreMatcher {
template <typename Container> operator Matcher<Container>() {
typedef typename std::remove_const<
typename std::remove_reference<Container>::type>::type ::value_type
Element;
typedef std::vector<Matcher<Element>> MatcherVec;
MatcherVec matchers;
matchers.reserve(std::tuple_size<MatcherTuple>::value);
return 0;
}
};
} // namespace internal
} // namespace testing
template <typename> struct Nullable {
template <typename V> Nullable(V);
};
void f(testing::internal::ElementsAreMatcher<std::tuple<>> m) {
testing::SafeMatcherCast<std::vector<std::variant<Nullable<int>>>>(m);
}
```
```
In file included from <source>:1:
/opt/compiler-explorer/clang-trunk-20250723/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-20250723/bin/../include/c++/v1/variant:1192:35:
note: while substituting prior template arguments into non-type template
parameter [with _Arg = testing::Matcher<std::variant<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-20250723/bin/../include/c++/v1/__type_traits/invoke.h:71:69:
note: while substituting deduced template arguments into function template
'variant' [with _Arg = testing::Matcher<std::variant<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-20250723/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-20250723/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-20250723/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-20250723/bin/../include/c++/v1/variant:1192:35:
note: (skipping 2539 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/clang-trunk-20250723/bin/../include/c++/v1/__vector/vector.h:845:8:
note: in instantiation of function template specialization
'std::__uninitialized_allocator_relocate<std::allocator<testing::Matcher<std::variant<Nullable<int>>>>,
testing::Matcher<std::variant<Nullable<int>>> *>' requested here
845 | std::__uninitialized_allocator_relocate(
| ^
/opt/compiler-explorer/clang-trunk-20250723/bin/../include/c++/v1/__vector/vector.h:1101:5:
note: in instantiation of member function
'std::vector<testing::Matcher<std::variant<Nullable<int>>>>::__swap_out_circular_buffer'
requested here
1101 | __swap_out_circular_buffer(__v);
| ^
<source>:20:14: note: in instantiation of member function
'std::vector<testing::Matcher<std::variant<Nullable<int>>>>::reserve' requested
here
20 | matchers.reserve(std::tuple_size<MatcherTuple>::value);
| ^
<source>:10:10: note: in instantiation of function template specialization
'testing::internal::ElementsAreMatcher<std::tuple<>>::operator
Matcher<std::vector<std::variant<Nullable<int>>>>' requested here
10 | return polymorphic_matcher_or_value;
| ^
<source>:30:12: note: in instantiation of function template specialization
'testing::SafeMatcherCast<std::vector<std::variant<Nullable<int>>>,
testing::internal::ElementsAreMatcher<std::tuple<>>>' requested here
30 | testing::SafeMatcherCast<std::vector<std::variant<Nullable<int>>>>(m);
| ^
```
@philnik777 PTAL
https://github.com/llvm/llvm-project/pull/116709
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