[Bug c++/107124] New: Reference template parameter refers to a temporary object

[Bug c++/107124] New: Reference template parameter refers to a temporary object

[fchelnokov at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=107124

            Bug ID: 107124
           Summary: Reference template parameter refers to a temporary
                    object
           Product: gcc
           Version: 12.2.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: fchelnokov at gmail dot com
  Target Milestone: ---

The following program is accepted by Clang:

template<int>
struct A {};

template<const int & I>
constexpr int f(A<I>) { return 0; }

template<class T>
constexpr int f(T) { return 1; }

static_assert( f(A<0>{}) == 1 );

But in GCC static assertion fails, because f(A<I>) overload is preferred.
Online demo: https://gcc.godbolt.org/z/9hj4WoT7b

But this is illegal by http://eel.is/c++draft/temp.arg.nontype#3.1, since const
int & I cannot refer to a temporary int 0.

[Bug c++/107124] Reference template parameter refers to a temporary object

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=107124

--- Comment #1 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
MSVC also invokes the static assert but ICC accepts it.


Note I think clang/ICC get the following wrong too:
template<int>
struct A {};
template<const int & I>
constexpr int f(A<I>) { return 0; }
template<class T>
constexpr int f(T) { return 1; }
const int tt = 0;
static_assert( f(A<tt>{}) == 1 );

I don't think any compiler implements this correct either ...

[Bug c++/107124] Reference template parameter refers to a temporary object

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=107124

--- Comment #2 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
Here is one which GCC/clang agree and accepts but MSVC/ICC reject (even before
the static assert):
template<int>
struct A {};
template<const int & I>
constexpr int f(A<I>) { return 0; }
template<class T>
constexpr int f(T) { return 1; }
const int tt = 0;
static_assert( f<tt>(A<tt>{}) == 0, "" );
---- CUT ----

So it looks like it is more complex than what I even orginally though.

[Bug c++/107124] Reference template parameter refers to a temporary object

[fchelnokov at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=107124

--- Comment #3 from Fedor Chelnokov <fchelnokov at gmail dot com> ---
The latter example is indeed a bug in MSVC:
https://developercommunity.visualstudio.com/t/Cannot-find-template-function-with-expli/1672180

And here is the related discussion according the original issue:
https://stackoverflow.com/a/73921590/7325599

[Bug c++/107124] Reference template parameter refers to a temporary object

[mpolacek at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=107124

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |mpolacek at gcc dot gnu.org
     Ever confirmed|0                           |1
   Last reconfirmed|                            |2022-10-31
             Status|UNCONFIRMED                 |NEW

--- Comment #4 from Marek Polacek <mpolacek at gcc dot gnu.org> ---
Confirmed.