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From: "hewillk at gmail dot com" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug c++/102881] gcc totally broken when trailing return type
 combine with decltype lambda
Date: Thu, 21 Oct 2021 14:41:26 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D102881

--- Comment #1 from =E5=BA=B7=E6=A1=93=E7=91=8B <hewillk at gmail dot com> =
---
(In reply to =E5=BA=B7=E6=A1=93=E7=91=8B from comment #0)
> template<int N>
> using R =3D decltype([] { return 0; }());
>=20
> template <int N>
> auto you_can_see_me() -> R<N> {
>   return {};
> }
>=20
> int main() {
>   you_can_see_me<0>();
> }
>=20
> GCC rejects with:
>=20
> <source>:10:3: error: 'you_can_see_me' was not declared in this scope
>    10 |   you_can_see_me<0>();
>       |   ^~~~~~~~~~~~~~~~~
>=20
> https://godbolt.org/z/7WEsjjscT

Even broken when there is no trailing return type:

#include <concepts>

template<class>
struct O {};

template<std::size_t>
using R =3D decltype([] { return O<int>{}; }());

template <int N>
constexpr R<N> get_variant() { return {}; }

static_assert(std::same_as<R<0>, O<int>>);
auto x =3D get_variant<0>();
static_assert(!std::same_as<decltype(x), O<int>>);
static_assert(std::same_as<decltype(x), int>);

https://godbolt.org/z/4bKccTEe5=