https://gcc.gnu.org/bugzilla/show_bug.cgi?id=126615
Bug ID: 126615
Summary: [C++20] {} accepted as a template-argument for a
template template parameter with a placeholder type
Product: gcc
Version: 16.1.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: c++
Assignee: unassigned at gcc dot gnu.org
Reporter: fchelnokov at gmail dot com
Target Milestone: ---
```
#include <initializer_list>
template <int N> struct A {};
template <template <auto> class TT> using G = TT<{}>;
using X = G<A>; // GCC accepts
```
auto cannot be deduced from an empty braced-init-list, so this should be
rejected. Clang, EDG and MSVC reject it; GCC also rejects the same parameter
written directly (template <auto V> struct Au {}; using X = Au<{}>;).
Dropping #include <initializer_list> makes GCC reject it too, with deducing
from brace-enclosed initializer list requires '#include <initializer_list>' -
yet no std::initializer_list takes part in the accepted path: TT becomes A, and
{} copy-initializes its int parameter.
Online demo: https://gcc.godbolt.org/z/jE4Tc1bb4
