[Bug c++/116440] New: [C++20] std::tuple<std::tuple<std::any>> does not compile

[Bug c++/116440] New: [C++20] std::tuple<std::tuple<std::any>> does not compile

[tianchengyu at tencent dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=116440

            Bug ID: 116440
           Summary: [C++20] std::tuple<std::tuple<std::any>> does not
                    compile
           Product: gcc
           Version: 14.2.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: tianchengyu at tencent dot com
  Target Milestone: ---

The following code (minimized reproduction) does not compile since gcc 14
(tested on 14.1.0 and 14.2.0), also failed on trunk (as of 20240821).

template <typename T>
using TupleTuple = std::tuple<std::tuple<T>>; // can be replaced with
std::tuple<std::tuple<...>> in following code, just to make it shorter.

struct EmbedAny {
    std::any content;
};

// Does not compile: struct with only one std::any field
static_assert(std::is_copy_constructible<TupleTuple<EmbedAny>>::value);
// Does not compile: just std::any in nested std::tuple
static_assert(std::is_copy_constructible<TupleTuple<std::any>>::value);

However, if we add anything before the std::any field, it compiles fine (even a
zero-sized array!):

struct EmbedAnyWithZeroSizeArray {
    void* pad[0];  // or, e.g. int pad; would also compile
    std::any content;
};

// Compiles: add a zero-sized (or any size) field **before** std::any field.
Adding the field after std::any does not work.
static_assert(std::is_copy_constructible<TupleTuple<EmbedAnyWithZeroSizeArray>>::value);

This only fails on gcc 14+ with -std=c++20 or higher. On older gcc versions
with -std=c++20 or gcc 14+ with -std=c++17 it compiles fine.

I also observed that clang trunk failed to compile with similar errors, but
clang 18 works fine, too.

Godbolt sample: https://godbolt.org/z/Gds4ar716

[Bug c++/116440] [C++20] std::tuple<std::tuple<std::any>> does not compile

[tianchengyu at tencent dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=116440

--- Comment #1 from tianchengyu(余添诚) <tianchengyu at tencent dot com> ---
Note that in the sample I used static_assert(std::is_copy_constructible<...>)
for testing, but it is not that is_copy_constructible is false. The
std::tuple<std::tuple<std::any>>> type simply does not compile at all.

Unsure if this is a c++ frontend issue or a libstdc++ issue.

[Bug c++/116440] [14/15 Regression] [C++20] std::tuple<std::tuple<std::any>> does not compile

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=116440

--- Comment #2 from Jonathan Wakely <redi at gcc dot gnu.org> ---
(In reply to Tiancheng Yu from comment #0)
> This only fails on gcc 14+ with -std=c++20 or higher. On older gcc versions
> with -std=c++20 or gcc 14+ with -std=c++17 it compiles fine.

GCC 14 has new definitions of std::tuple constructors that are used when C++20
is enabled, controlled by:

#if __cpp_concepts && __cpp_consteval && __cpp_conditional_explicit

> I also observed that clang trunk failed to compile with similar errors,

Because it's using the new libstdc++ headers.

> but
> clang 18 works fine, too.

Almost certainly because it's using older libstdc++ headers, probably from GCC
13.

It's not very useful to just say something works with clang on godbolt without
considering 
whether it's using libc++ or libstdc++, and for the latter, *which version* of
libstdc++.

[Bug c++/116440] [14/15 Regression] [C++20] std::tuple<std::tuple<std::any>> does not compile

[tianchengyu at tencent dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=116440

--- Comment #3 from Tiancheng Yu <tianchengyu at tencent dot com> ---
(In reply to Jonathan Wakely from comment #2)
> It's not very useful to just say something works with clang on godbolt
> without considering 
> whether it's using libc++ or libstdc++, and for the latter, *which version*
> of libstdc++.

Sorry for not being clear. clang does not compile with libstdc++ (trunk, as of
20240821), but works fine with libc++.

[Bug c++/116440] [14/15 Regression] [C++20] std::tuple<std::tuple<std::any>> does not compile

[tianchengyu at tencent dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=116440

--- Comment #4 from Tiancheng Yu <tianchengyu at tencent dot com> ---
on clang 18.1, works with both libstdc++ (from GCC 13.2.0) and libc++.

on clang trunk, works with libc++ (trunk) but not libstdc++ (trunk).