https://gcc.gnu.org/bugzilla/show_bug.cgi?id=114138
Bug ID: 114138
Summary: [c++2b] ICE on valid code using `auto(expr)`
DECAY-COPY
Product: gcc
Version: 14.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: c++
Assignee: unassigned at gcc dot gnu.org
Reporter: eric.niebler at gmail dot com
Target Milestone: ---
Compile the following with -c++=2b:
```
namespace std {
template <class T>
T&& declval() noexcept requires true;
template <class>
void declval() noexcept;
namespace detail {
struct none_such;
template <class>
using none_such_t = none_such;
template <class T>
extern const none_such_t<T> _getter_for;
template <class T>
using _decay_t = decltype(auto(declval<T>()));
static_assert(__is_same_as(_decay_t<void>, void));
}
template <const auto& Fn, class... Args>
using _result_of_t = decltype(Fn(declval<Args>()...));
template <unsigned I, class Tuple>
using tuple_element_t =
_result_of_t<detail::_getter_for<detail::_decay_t<Tuple>>, char(*)[I+1],
Tuple>;
template <class First, class Second>
struct pair {
First first;
Second second;
};
template <class>
inline constexpr bool _is_pair = false;
template <class First, class Second>
inline constexpr bool _is_pair<pair<First, Second>> = true;
template <class T>
concept Pair = _is_pair<decltype(auto(std::declval<T>()))>;
template <unsigned I, Pair P>
requires (I <= 1)
decltype(auto) get(P&& p) noexcept {
if constexpr (I == 0) {
return (static_cast<P&&>(p).first);
} else {
return (static_cast<P&&>(p).second);
}
}
namespace detail {
inline constexpr auto _pair_getter =
[]<unsigned J, class Pair>(char(*)[J], Pair&& p) noexcept ->
decltype(auto) {
return std::get<J-1>(static_cast<Pair&&>(p));
};
template <class First, class Second>
inline constexpr auto _getter_for<pair<First, Second>> = _pair_getter;
}
}
int main() {
static_assert(__is_same_as(int&, std::tuple_element_t<0, std::pair<int,
float>&>));
static_assert(__is_same_as(float&&, std::tuple_element_t<1, std::pair<int,
float>&&>));
}
```
Result:
```
<source>: In substitution of 'template<unsigned int I, class Tuple> using
std::tuple_element_t = std::_result_of_t<_getter_for<decltype
((auto)(declval<Tuple>()))>, char (*)[(I + 1)], Tuple> [with unsigned int I =
0; Tuple = std::pair<int, float>&]':
<source>:71:82: required from here
71 | static_assert(__is_same_as(int&, std::tuple_element_t<0,
std::pair<int, float>&>));
|
^
<source>:21:31: internal compiler error: in tsubst, at cp/pt.cc:16367
21 | using _decay_t = decltype(auto(declval<T>()));
| ^~~~~~~~~~~~~~~~~~
0x265412c internal_error(char const*, ...)
???:0
0xa548c3 fancy_abort(char const*, int, char const*)
???:0
0xc99307 tsubst(tree_node*, tree_node*, int, tree_node*)
???:0
0xc9f3c9 tsubst_template_args(tree_node*, tree_node*, int, tree_node*)
???:0
0xc9f3c9 tsubst_template_args(tree_node*, tree_node*, int, tree_node*)
???:0
0xc98a19 tsubst(tree_node*, tree_node*, int, tree_node*)
???:0
0xc80873 instantiate_template(tree_node*, tree_node*, int)
???:0
0xc9a0fc tsubst(tree_node*, tree_node*, int, tree_node*)
???:0
0xc973cc lookup_template_class(tree_node*, tree_node*, tree_node*, tree_node*,
int, int)
???:0
0xcd370f finish_template_type(tree_node*, tree_node*, int)
???:0
0xc577da c_parse_file()
???:0
0xdaba19 c_common_parse_file()
???:0
Please submit a full bug report, with preprocessed source (by using
-freport-bug).
Please include the complete backtrace with any bug report.
See <https://gcc.gnu.org/bugs/> for instructions.
Compiler returned: 1
```
See https://godbolt.org/z/r13Yff5bM
