[Bug libstdc++/103879] New: error: accessing value of variant::_Copy_ctor_base through a 'const variant<string>' glvalue in a constant expression

[Bug libstdc++/103879] New: error: accessing value of variant::_Copy_ctor_base through a 'const variant<string>' glvalue in a constant expression

[hewillk at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=103879

            Bug ID: 103879
           Summary: error: accessing value of variant::_Copy_ctor_base
                    through a 'const variant<string>' glvalue in a
                    constant expression
           Product: gcc
           Version: 12.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: libstdc++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: hewillk at gmail dot com
  Target Milestone: ---

From
https://stackoverflow.com/questions/70541636/stdvariant-of-stdstring-inside-any-class-in-constexpr-context-fails-to-compi

#include <string>
#include <variant>

struct S {
  std::variant<std::string> v;
};

constexpr int func() {
  S s{"str"};
  return 0;
}

constexpr int x = func();

I think this should be well-formed.

[Bug libstdc++/103879] error: accessing value of variant::_Copy_ctor_base through a 'const variant<string>' glvalue in a constant expression

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=103879

--- Comment #1 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
clang rejects it also (with their libc++):

<source>:10:5: error: variable of non-literal type 'S' cannot be defined in a
constexpr function
  S s{"str"};
    ^
<source>:6:29: note: 'S' is not literal because it has data member 'v' of
non-literal type 'std::variant<std::string>' (aka 'variant<basic_string<char,
char_traits<char>, allocator<char>>>')
  std::variant<std::string> v;
                            ^

[Bug libstdc++/103879] error: accessing value of variant::_Copy_ctor_base through a 'const variant<string>' glvalue in a constant expression

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=103879

--- Comment #2 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
MSVC also rejects it:
example.cpp
<source>(14): error C2131: expression did not evaluate to a constant
<source>(10): note: failure was caused by call of undefined function or one not
declared 'constexpr'
<source>(10): note: see usage of 'S::~S'

[Bug libstdc++/103879] error: accessing value of variant::_Copy_ctor_base through a 'const variant<string>' glvalue in a constant expression

[lhlaurini at hotmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=103879

Luiz Henrique Laurini <lhlaurini at hotmail dot com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |lhlaurini at hotmail dot com

--- Comment #3 from Luiz Henrique Laurini <lhlaurini at hotmail dot com> ---
(In reply to Andrew Pinski from comment #2)
> MSVC also rejects it:
> example.cpp
> <source>(14): error C2131: expression did not evaluate to a constant
> <source>(10): note: failure was caused by call of undefined function or one
> not declared 'constexpr'
> <source>(10): note: see usage of 'S::~S'

It does compile with MSVC v19.latest: https://godbolt.org/z/McKjfdP5Y.

[Bug libstdc++/103879] error: accessing value of variant::_Copy_ctor_base through a 'const variant<string>' glvalue in a constant expression

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=103879

--- Comment #4 from Jonathan Wakely <redi at gcc dot gnu.org> ---
(In reply to Andrew Pinski from comment #1)
> clang rejects it also (with their libc++):

Libc++ doesn't support constexpr std::string, and I think it's missing some DRs
to std::variant as well. So I wouldn't use it to confirm anything like this.

[Bug libstdc++/103879] error: accessing value of variant::_Copy_ctor_base through a 'const variant<string>' glvalue in a constant expression

[ppalka at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=103879

Patrick Palka <ppalka at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
     Ever confirmed|0                           |1
                 CC|                            |ppalka at gcc dot gnu.org
           Assignee|unassigned at gcc dot gnu.org      |ppalka at gcc dot gnu.org
             Status|UNCONFIRMED                 |ASSIGNED
   Last reconfirmed|                            |2022-01-04

--- Comment #5 from Patrick Palka <ppalka at gcc dot gnu.org> ---
Looks like this is a frontend bug, the constexpr engine can't resolve the
indirection (where the D.NNNNN correspond to base class subobjects, the
innermost of which is the _Variant_storage base)

  *(struct variant *) &s.v.D.47079.D.46560.D.46478.D.46346.D.46232.D.46136

into just

  s.v

[Bug c++/103879] error: accessing value of variant::_Copy_ctor_base through a 'const variant<string>' glvalue in a constant expression

[ppalka at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=103879

--- Comment #6 from Patrick Palka <ppalka at gcc dot gnu.org> ---
Reduced C++14 rejects-valid testcase:

struct A {
  int n = 42;
};

struct B : A { };

struct C {
  B b;
};

constexpr int f() {
  C c;
  A& a = static_cast<A&>(c.b);
  B& b = static_cast<B&>(a);
  return b.n;
}

static_assert(f() == 42, "");

[Bug c++/103879] error: accessing value of variant::_Copy_ctor_base through a 'const variant<string>' glvalue in a constant expression

[hewillk at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=103879

--- Comment #7 from 康桓瑋 <hewillk at gmail dot com> ---
(In reply to Patrick Palka from comment #6)
> Reduced C++14 rejects-valid testcase:
> 
> struct A {
>   int n = 42;
> };
> 
> struct B : A { };
> 
> struct C {
>   B b;
> };
> 
> constexpr int f() {
>   C c;
>   A& a = static_cast<A&>(c.b);
>   B& b = static_cast<B&>(a);
>   return b.n;
> }
> 
> static_assert(f() == 42, "");

Is this the same issue?

#include <string>
#include <ranges>

int main() {
  using namespace std::string_literals;
  static_assert(std::ranges::distance(
      std::views::iota(0, 2) | std::ranges::views::transform([](int) { return
""s; })
                             | std::ranges::views::join));
}

https://godbolt.org/z/8nve43zvW

[Bug c++/103879] error: accessing value of variant::_Copy_ctor_base through a 'const variant<string>' glvalue in a constant expression

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=103879

--- Comment #8 from Jonathan Wakely <redi at gcc dot gnu.org> ---
Yes, looks like it.

[Bug c++/103879] error: accessing value of variant::_Copy_ctor_base through a 'const variant<string>' glvalue in a constant expression

[ppalka at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=103879

--- Comment #9 from Patrick Palka <ppalka at gcc dot gnu.org> ---
That one unfortunately seems to be an unrelated constexpr bug.  Mind opening a
separate PR?

I'm reducing the testcase..

[Bug c++/103879] error: accessing value of variant::_Copy_ctor_base through a 'const variant<string>' glvalue in a constant expression

[hewillk at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=103879

--- Comment #10 from 康桓瑋 <hewillk at gmail dot com> ---
(In reply to Patrick Palka from comment #9)
> That one unfortunately seems to be an unrelated constexpr bug.  Mind opening
> a separate PR?
> 
> I'm reducing the testcase..

Reported PR103924.

[Bug c++/103879] error: accessing value of variant::_Copy_ctor_base through a 'const variant<string>' glvalue in a constant expression

[cvs-commit at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=103879

--- Comment #11 from CVS Commits <cvs-commit at gcc dot gnu.org> ---
The master branch has been updated by Patrick Palka <ppalka@gcc.gnu.org>:

https://gcc.gnu.org/g:ab36b554bd90e8db279d13b133369118814f13fb

commit r12-6425-gab36b554bd90e8db279d13b133369118814f13fb
Author: Patrick Palka <ppalka@redhat.com>
Date:   Mon Jan 10 14:57:54 2022 -0500

    c++: constexpr base-to-derived conversion with offset 0 [PR103879]

    r12-136 made us canonicalize an object/offset pair with negative offset
    into one with a nonnegative offset, by iteratively absorbing the
    innermost component into the offset and stopping as soon as the offset
    becomes nonnegative.

    This patch strengthens this transformation by making it keep on absorbing
    even if the offset is already 0 as long as the innermost component is at
    position 0 (and thus absorbing doesn't change the offset).  This lets us
    accept the two constexpr testcases below, which we'd previously reject
    essentially because cxx_fold_indirect_ref would be unable to resolve
    *(B*)&b.D123 (where D123 is the base A subobject at position 0) to just b.

            PR c++/103879

    gcc/cp/ChangeLog:

            * constexpr.c (cxx_fold_indirect_ref): Split out object/offset
            canonicalization step into a local lambda.  Strengthen it to
            absorb more components at position 0.  Use it before both calls
            to cxx_fold_indirect_ref_1.

    gcc/testsuite/ChangeLog:

            * g++.dg/cpp1y/constexpr-base2.C: New test.
            * g++.dg/cpp1y/constexpr-base2a.C: New test.

[Bug c++/103879] error: accessing value of variant::_Copy_ctor_base through a 'const variant<string>' glvalue in a constant expression

[ppalka at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=103879

Patrick Palka <ppalka at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|ASSIGNED                    |RESOLVED
         Resolution|---                         |FIXED
   Target Milestone|---                         |12.0

--- Comment #12 from Patrick Palka <ppalka at gcc dot gnu.org> ---
Fixed for GCC 12

[Bug c++/103879] error: accessing value of variant::_Copy_ctor_base through a 'const variant<string>' glvalue in a constant expression

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=103879

Andrew Pinski <pinskia at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |petr.azmanov at wartsila dot com

--- Comment #13 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
*** Bug 107165 has been marked as a duplicate of this bug. ***