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From: "namniav at gmail dot com" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug c++/105644] ICE in a fold expression with a requires
 expression: in iterative_hash_template_arg, at cp/pt.cc:1805
Date: Fri, 11 Aug 2023 00:19:30 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D105644

--- Comment #2 from Namniav <namniav at gmail dot com> ---
Now Clang also crashes on the original
example(https://github.com/llvm/llvm-project/issues/64607), although its ca=
use
may be different to GCC's.

I can't remember why I used `get` instead of `std::get` in original code
because I didn't really know how ADL finds function templates. The issue st=
ill
exists with `std::get`.

Here is a modified and GCC-only reduced version:
https://godbolt.org/z/MMYMrGE3z

```c++
template<typename T, int I>
concept C =3D (sizeof(I) + sizeof(T) > 0);

template<typename T, int...I>
constexpr bool check() {
    return ((requires(T t) {
                { (I, t) } -> C<I>;
            }) && ...);
}

template<class T>
concept tuple_like =3D check<T, 0>();

static_assert(tuple_like<int>);
```

Noted that GCC doesn't consider using `I` in `-> C<I>` as using a parameter
pack, I have to use it in comma expression `(I, t)` to get a fold expressio=
n,
otherwise GCC complains "operand of fold expression has no unexpanded param=
eter
packs". I suspect that GCC has problem with such case. It some how explains,
for GCC only, why replacing `std::tuple_element_t<I, T>&` with `int&` "fixe=
s"
the issue.=