[Bug c++/114994] New: fmtlib named argument compiler error introduced in g++-14.1

[Bug c++/114994] New: fmtlib named argument compiler error introduced in g++-14.1

[andrew.corrigan at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=114994

            Bug ID: 114994
           Summary: fmtlib named argument compiler error introduced in
                    g++-14.1
           Product: gcc
           Version: 14.1.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: andrew.corrigan at gmail dot com
  Target Milestone: ---

Demo: https://godbolt.org/z/oachhYKcT

First reported to https://github.com/fmtlib/fmt/issues/3953.  The fmtlib author
believes the error below is a compiler bug.  Using fmtlib's named arguments
inside a generic lambda stopped working as of g++-14.1.  The reproducer below
compiles on every other compiler I've tried (earlier versions of g++, clang++,
intel, and msvc)


```
#include <iostream>

#define FMT_HEADER_ONLY

#include <fmt/format.h>
using namespace fmt::literals;

int main()
{
    auto test = [&](auto a)
    {
        return fmt::format("{foo} {bar}", "foo"_a="foo", "bar"_a="bar");
    };

    std::cout << test(1) << std::endl;

    return 0;
}
```

Error:
```
<source>:12:50: error: cannot bind non-const lvalue reference of type
'fmt::v10::detail::named_arg<char, const char (&)[4]>&' to an rvalue of type
'fmt::v10::detail::named_arg<char, const char (&)[4]>'
   12 |         return fmt::format("{foo} {bar}", "foo"_a="foo",
"bar"_a="bar");
      |                                           ~~~~~~~^~~~~~
```

Changing `auto a` to `int a` works around the error.

Thank you!

[Bug c++/114994] fmtlib named argument compiler error introduced in g++-14.1

[mpolacek at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=114994

Marek Polacek <mpolacek at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
           Keywords|                            |needs-reduction
                 CC|                            |mpolacek at gcc dot gnu.org,
                   |                            |ppalka at gcc dot gnu.org

--- Comment #1 from Marek Polacek <mpolacek at gcc dot gnu.org> ---
Looks like it started with r14-4111:

commit 6e92a6a2a72d3b7a5e1b29042d8a6a43fe1085aa
Author: Patrick Palka <ppalka@redhat.com>
Date:   Mon Sep 18 14:47:52 2023 -0400

    c++: non-dependent assignment checking [PR63198, PR18474]

[Bug c++/114994] fmtlib named argument compiler error introduced in g++-14.1

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=114994

--- Comment #2 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
Note lambdas is not need can happen in any template function:
```
#include <iostream>

#define FMT_HEADER_ONLY

#include <fmt/format.h>
using namespace fmt::literals;

template<int T>
auto test(int a)
{
        return fmt::format("{foo} {bar}", "foo"_a="foo", "bar"_a="bar");
}
auto t = test<1>(1);
```

[Bug c++/114994] fmtlib named argument compiler error introduced in g++-14.1

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=114994

--- Comment #3 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
Trying my hand at reducing this slightly.

[Bug c++/114994] [14/15 Regression] fmtlib named argument compiler error introduced in g++-14.1

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=114994

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
            Summary|fmtlib named argument       |[14/15 Regression] fmtlib
                   |compiler error introduced   |named argument compiler
                   |in g++-14.1                 |error introduced in
                   |                            |g++-14.1
   Target Milestone|---                         |14.2
     Ever confirmed|0                           |1
           Keywords|needs-reduction             |rejects-valid
   Last reconfirmed|                            |2024-05-08
             Status|UNCONFIRMED                 |NEW

--- Comment #4 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
Reduced testcase:
```
typedef decltype(sizeof(1)) size_t;
struct udl_arg {
  const char *str;
  template <typename T> auto operator=(T &&value) const -> int {}
};
constexpr auto operator""_a(const char *s, size_t) -> udl_arg
{
  return {""};
}
template <typename T> void h(T &&);
template<int T>
int test(int a)
{
        h("t"_a="t");
}
auto t = test<1>(1);
````

[Bug c++/114994] [14/15 Regression] fmtlib named argument compiler error introduced in g++-14.1

[ppalka at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=114994

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
           Assignee|unassigned at gcc dot gnu.org      |ppalka at gcc dot gnu.org
             Status|NEW                         |ASSIGNED

--- Comment #5 from Patrick Palka <ppalka at gcc dot gnu.org> ---
A bit more reduced, demostrating it's not specific to UDLs:

struct udl_arg {
  udl_arg operator=(int);
};

void g(udl_arg&&);

template<class T>
void h() {
  udl_arg x;
  g(x=42);
}

int main() {
  h<int>();
}

[Bug c++/114994] [14/15 Regression] fmtlib named argument compiler error introduced in g++-14.1

[rguenth at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=114994

Richard Biener <rguenth at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
           Priority|P3                          |P2

[Bug c++/114994] [14/15 Regression] fmtlib named argument compiler error introduced in g++-14.1

[cvs-commit at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=114994

--- Comment #6 from GCC 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:c6cc6d4741a880109c4e0e64d5a189687fb526f6

commit r15-498-gc6cc6d4741a880109c4e0e64d5a189687fb526f6
Author: Patrick Palka <ppalka@redhat.com>
Date:   Tue May 14 22:55:16 2024 -0400

    c++: lvalueness of non-dependent assignment expr [PR114994]

    r14-4111-g6e92a6a2a72d3b made us check non-dependent simple assignment
    expressions ahead of time and give them a type, as was already done for
    compound assignments.  Unlike for compound assignments however, if a
    simple assignment resolves to an operator overload we represent it as a
    (typed) MODOP_EXPR instead of a CALL_EXPR to the selected overload.
    (I reckoned this was at worst a pessimization -- we'll just have to repeat
    overload resolution at instantiatiation time.)

    But this turns out to break the below testcase ultimately because
    MODOP_EXPR (of non-reference type) is always treated as an lvalue
    according to lvalue_kind, which is incorrect for the MODOP_EXPR
    representing x=42.

    We can fix this by representing such class assignment expressions as
    CALL_EXPRs as well, but this turns out to require some tweaking of our
    -Wparentheses warning logic and may introduce other fallout making it
    unsuitable for backporting.

    So this patch instead fixes lvalue_kind to consider the type of a
    MODOP_EXPR representing a class assignment.

            PR c++/114994

    gcc/cp/ChangeLog:

            * tree.cc (lvalue_kind) <case MODOP_EXPR>: For a class
            assignment, consider the result type.

    gcc/testsuite/ChangeLog:

            * g++.dg/template/non-dependent32.C: New test.

    Reviewed-by: Jason Merrill <jason@redhat.com>

[Bug c++/114994] [14/15 Regression] fmtlib named argument compiler error introduced in g++-14.1

[cvs-commit at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=114994

--- Comment #7 from GCC Commits <cvs-commit at gcc dot gnu.org> ---
The releases/gcc-14 branch has been updated by Patrick Palka
<ppalka@gcc.gnu.org>:

https://gcc.gnu.org/g:b3399b445ba7495b0479d43f2389e64d48de870e

commit r14-10220-gb3399b445ba7495b0479d43f2389e64d48de870e
Author: Patrick Palka <ppalka@redhat.com>
Date:   Tue May 14 22:55:16 2024 -0400

    c++: lvalueness of non-dependent assignment expr [PR114994]

    r14-4111-g6e92a6a2a72d3b made us check non-dependent simple assignment
    expressions ahead of time and give them a type, as was already done for
    compound assignments.  Unlike for compound assignments however, if a
    simple assignment resolves to an operator overload we represent it as a
    (typed) MODOP_EXPR instead of a CALL_EXPR to the selected overload.
    (I reckoned this was at worst a pessimization -- we'll just have to repeat
    overload resolution at instantiatiation time.)

    But this turns out to break the below testcase ultimately because
    MODOP_EXPR (of non-reference type) is always treated as an lvalue
    according to lvalue_kind, which is incorrect for the MODOP_EXPR
    representing x=42.

    We can fix this by representing such class assignment expressions as
    CALL_EXPRs as well, but this turns out to require some tweaking of our
    -Wparentheses warning logic and may introduce other fallout making it
    unsuitable for backporting.

    So this patch instead fixes lvalue_kind to consider the type of a
    MODOP_EXPR representing a class assignment.

            PR c++/114994

    gcc/cp/ChangeLog:

            * tree.cc (lvalue_kind) <case MODOP_EXPR>: For a class
            assignment, consider the result type.

    gcc/testsuite/ChangeLog:

            * g++.dg/template/non-dependent32.C: New test.

    Reviewed-by: Jason Merrill <jason@redhat.com>
    (cherry picked from commit c6cc6d4741a880109c4e0e64d5a189687fb526f6)

[Bug c++/114994] [14/15 Regression] fmtlib named argument compiler error introduced in g++-14.1

[ppalka at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=114994

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|ASSIGNED                    |RESOLVED
         Resolution|---                         |FIXED

--- Comment #8 from Patrick Palka <ppalka at gcc dot gnu.org> ---
Fixed for GCC 14.2, thanks for the bug report.