[Bug c++/108234] New: GCC accepts invalid program involving nontype template parameter of auto with non constexpr variable

[Bug c++/108234] New: GCC accepts invalid program involving nontype template parameter of auto with non constexpr variable

[jlame646 at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=108234

            Bug ID: 108234
           Summary: GCC accepts invalid program involving nontype template
                    parameter of auto with non constexpr variable
           Product: gcc
           Version: 13.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: jlame646 at gmail dot com
  Target Milestone: ---

GCC trunk accepts the following invalid program:
https://godbolt.org/z/h5q7589Tr

```
template <int&> struct X{};

template <auto V> int func(X<V>) { return 1; }

int i;
int main() { return func(X<i>{}); }

```

Clang rejects the above program while gcc and msvc accepts it.

[Bug c++/108234] GCC accepts invalid program involving nontype template parameter of auto with non constexpr variable

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=108234

--- Comment #1 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
If I change func to:
template <auto &V> int func(X<V>) {  return 1; }

And then clang accepts it.
The question becomes what can be deduced as the type of V.

Here is a slightly different testcase:
```
template <int&> struct X{};

template <typename T>
void ff(){};

template <typename T, T V> int func(X<V>) {  return 1; }

int i;
int main() { return func(X<i>{}); }
```
Note GCC rejects the above testcase for C++14 but accepts it for C++17.
So I am not 100% this is invalid and clang just does not implement some rule of
C++17 correctly dealing with reference types ...

[Bug c++/108234] GCC accepts invalid program involving nontype template parameter of auto with non constexpr variable

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=108234

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|UNCONFIRMED                 |RESOLVED
         Resolution|---                         |INVALID

--- Comment #2 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
https://www.open-std.org/jtc1/sc22/wg21/docs/papers/2016/p0127r2.html

Looks like clang does not implement this paper correctly for reference types
...

Modified testcase from that paper using reference types, C++17 should reject
this while c++14 will accept it:
template <const int &N> struct A;
template <typename T, T N> int foo(A<N> *) = delete;
void foo(void *);
constexpr int t = 1;
void bar(A<t> *p) {
  foo(p); // ill-formed; previously well-formed
}

[Bug c++/108234] GCC accepts invalid program involving nontype template parameter of auto with non constexpr variable

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=108234

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
           See Also|                            |https://github.com/llvm/llv
                   |                            |m-project/issues/40328

--- Comment #3 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
See:
https://github.com/llvm/llvm-project/issues/40328

[Bug c++/108234] GCC accepts invalid program involving nontype template parameter of auto with non constexpr variable

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=108234

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
           See Also|                            |https://github.com/llvm/llv
                   |                            |m-project/issues/38721

--- Comment #4 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
Also:
https://github.com/llvm/llvm-project/issues/38721