https://gcc.gnu.org/bugzilla/show_bug.cgi?id=121287
Bug ID: 121287
Summary: ICE: Segfault in tsubst_lambda_expr with decltype of
immediately-invoked templated lambda + variadic lambda
expansion (GCC 13+)
Product: gcc
Version: 15.1.1
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: c++
Assignee: unassigned at gcc dot gnu.org
Reporter: llh282000500 at gmail dot com
Target Milestone: ---
Created attachment 61991
--> https://gcc.gnu.org/bugzilla/attachment.cgi?id=61991&action=edit
Preprocessed source and crash trace are attached (generated with -freport-bug
on Arch Linux GCC 15.1.1).
This ICE affects all tested GCC versions >= 13 that support C++20, reproducible
on https://godbolt.org.
- Example 1 (ICE): https://godbolt.org/z/9EfjWEvTP
- Example 2 (ICE): https://godbolt.org/z/fM84Ex34j
The ICE occurs when two conditions are simultaneously true inside a templated
constexpr function:
1. A templated lambda with a parameter pack is immediately invoked and passed
to `decltype(...)`.
2. The result of that `decltype(...)` is used in another templated lambda with
parameter pack that also performs parameter pack expansion using the result.
This combination causes a crash in `tsubst_lambda_expr` during template
substitution.
## Minimal example:
1. decltype([]<...>{}()) + []<...>(){}()
```cpp
#include <variant>
#include <array>
template <std::size_t I>
struct SetObjIdx {};
// Must be a template
template <typename T = void>
constexpr auto makeNameToIdxVariantHashMap() {
constexpr auto N = 3U; // N arbitrary
// 1.
using Res = decltype([] <std::size_t... Idx> (std::index_sequence<Idx...>)
{
return std::variant<SetObjIdx<Idx>...>{};
} (std::make_index_sequence<N>{}));
// 2.
return [&] <std::size_t... Idx> (std::index_sequence<Idx...>) {
return std::array<Res, N>{
SetObjIdx<Idx>{}...
};
}(std::make_index_sequence<N>{});
}
int main() {
constexpr auto res = makeNameToIdxVariantHashMap();
(void)res;
return 0;
}
```
2. decltype([]<>{}()) + []<...>(){}()
```cpp
#include <array>
template <std::size_t I>
struct SetObjIdx { std::size_t _idx; };
// Must be a template
template <typename T = void>
constexpr auto makeNameToIdxVariantHashMap() {
constexpr auto N = 3U; // N arbitrary
// 1. Decltype combined with []<>(){}() template
using Res = decltype([] <std::size_t Idx> (std::index_sequence<Idx>) {
return SetObjIdx<Idx>{Idx};
}(std::index_sequence<N>{}));
// 2. It must be <...>Template
return [&] <std::size_t... Idx> (std::index_sequence<Idx...>) {
return std::array<Res, N>{
SetObjIdx<N>{Idx}...
};
}(std::make_index_sequence<N>{});
}
int main() {
constexpr auto res = makeNameToIdxVariantHashMap();
return 0;
}
```
Notes:
- Replacing the first lambda with a non-parameter-pack lambda avoids ICE.
- If the 'template<typename T=void>' function is deleted, it can be compiled
normally
- MSVC / Clang compilation is feasible
## Compile error message
1. decltype([]<...>{}()) + []<...>(){}()
```cpp
<source>: In instantiation of
'makeNameToIdxVariantHashMap<>()::<lambda(std::index_sequence<__var_indices
...>)> [with long unsigned int ...Idx = {0, 1, 2};
std::index_sequence<__var_indices ...> = std::integer_sequence<long unsigned
int, 0, 1, 2>]':
<source>:22:6: required from 'constexpr auto makeNameToIdxVariantHashMap()
[with T = void]'
18 | return [&] <std::size_t... Idx> (std::index_sequence<Idx...>) {
| ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
19 | return std::array<Res, N>{
| ~~~~~~~~~~~~~~~~~~~~~~~~~~
20 | SetObjIdx<Idx>{}...
| ~~~~~~~~~~~~~~~~~~~
21 | };
| ~~
22 | }(std::make_index_sequence<N>{});
| ~^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
<source>:26:53: required from here
26 | constexpr auto res = makeNameToIdxVariantHashMap();
| ~~~~~~~~~~~~~~~~~~~~~~~~~~~^~
<source>:13:26: internal compiler error: Segmentation fault
13 | using Res = decltype([] <std::size_t... Idx>
(std::index_sequence<Idx...>) {
|
^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
14 | return std::variant<SetObjIdx<Idx>...>{};
| ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
15 | } (std::make_index_sequence<N>{}));
| ~
0x2287785 diagnostic_context::diagnostic_impl(rich_location*,
diagnostic_metadata const*, diagnostic_option_id, char const*, __va_list_tag
(*) [1], diagnostic_t)
???:0
0x2298cd6 internal_error(char const*, ...)
???:0
0x989f38 template_parms_to_args(tree_node*)
???:0
0x9b8692 tsubst_lambda_expr(tree_node*, tree_node*, int, tree_node*)
???:0
0x9ab9bc tsubst(tree_node*, tree_node*, int, tree_node*)
???:0
0x9b2467 tsubst_template_args(tree_node*, tree_node*, int, tree_node*)
???:0
0x9ab435 tsubst(tree_node*, tree_node*, int, tree_node*)
???:0
0x99f62f instantiate_decl(tree_node*, bool, bool)
???:0
0x894821 maybe_instantiate_decl(tree_node*)
???:0
0x895b67 mark_used(tree_node*, int)
???:0
0x806f93 build_op_call(tree_node*, vec<tree_node*, va_gc, vl_embed>**, int)
???:0
0x9d8b67 finish_call_expr(tree_node*, vec<tree_node*, va_gc, vl_embed>**, bool,
bool, int)
???:0
0x99f62f instantiate_decl(tree_node*, bool, bool)
???:0
0x894821 maybe_instantiate_decl(tree_node*)
???:0
0x895b67 mark_used(tree_node*, int)
???:0
0x80330e build_new_function_call(tree_node*, vec<tree_node*, va_gc,
vl_embed>**, int)
???:0
0x9d8b3e finish_call_expr(tree_node*, vec<tree_node*, va_gc, vl_embed>**, bool,
bool, int)
???:0
0x981ebd c_parse_file()
???:0
0xa8b739 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.
```
