https://gcc.gnu.org/bugzilla/show_bug.cgi?id=126796
Bug ID: 126796
Summary: [c++26] "cannot index an empty pack" on uninstantiated
function
Product: gcc
Version: 16.2.1
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: c++
Assignee: unassigned at gcc dot gnu.org
Reporter: marcel at laverdet dot com
Target Milestone: ---
a.cc:
template <class...Types>
struct foo {
template <int Index>
auto get() const -> Types...[Index] { return {}; }
};
auto main() -> int {
auto ff = foo<>{};
return 0;
}
---
$ g++ --version
g++ (GCC) 16.2.0
Copyright (C) 2026 Free Software Foundation, Inc.
This is free software; see the source for copying conditions. There is NO
warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.
$ g++ -std=c++2c a.cc
a.cc: In instantiation of ‘struct foo<>’:
a.cc:8:18: required from here
8 | auto ff = foo<>{};
| ^
a.cc:4:14: error: cannot index an empty pack
4 | auto get() const -> Types...[Index] { return {}; }
| ^~~
$ clang++ -std=c++2c a.cc
// it's fine
---
Replacing with `auto get() const -> auto { return Types...[Index]{}; }` works.
