[Bug c++/103478] New: Possible regression in constexpr processing

[Bug c++/103478] New: Possible regression in constexpr processing

[fchelnokov at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=103478

            Bug ID: 103478
           Summary: Possible regression in constexpr processing
           Product: gcc
           Version: 10.1.0
            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 code is accepted by GCC 9.4 and rejected by GCC 10.1:
```
#include <array>
using namespace std;

template<size_t N> using string_literal_t = char[N];

template<class T> struct StrSize; ///< metafunction to get the size of string
literal alikes 

/// specialize StrSize for string literals
template<size_t N>
struct StrSize <string_literal_t<N>>{ static constexpr size_t value = N-1; };

/// template variable, just for convenience
template <class T>
constexpr size_t str_size = StrSize<T>::value;

/// now do the same but with constexpr function
template<class T>
constexpr auto strsize(const T&) noexcept-> decltype(str_size<T>) {
   return str_size<T>;
}

template<class S, size_t... Is>
constexpr auto test_helper(const S& s, index_sequence<Is...>) noexcept->
array<char, str_size<S>> {
   return {s[Is]...};
}

template<class S>
constexpr auto test(const S& s) noexcept-> decltype(auto) {
   return test_helper(s, make_index_sequence<strsize(s)>{});
}

auto main()-> int {
   static_assert(strsize("qwe") == 3, "");
   static_assert(noexcept(test("qwe")) == true, "");

   return 0;
}
```
Demo: https://gcc.godbolt.org/z/43TcY485x

Could you please check whether it is a regression?

Related discussion: https://stackoverflow.com/q/43072361/7325599

[Bug c++/103478] Possible regression in constexpr processing

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=103478

--- Comment #1 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
arguments to constexpr functions are not valid template arguments.

[Bug c++/103478] Possible regression in constexpr processing

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=103478

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
           Keywords|                            |diagnostic

--- Comment #2 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
That is:
template<class S>
constexpr auto test(const S& s) noexcept-> decltype(auto) {
   return test_helper(s, make_index_sequence<strsize(s)>{});
}

Is invalid.
Clang rejects it too but with a better diagnostic:
<source>:30:46: error: non-type template argument is not a constant expression
   return test_helper(s, make_index_sequence<strsize(s)>{});
                                             ^~~~~~~~~~
<source>:35:27: note: in instantiation of function template specialization
'test<char[4]>' requested here
   static_assert(noexcept(test("qwe")) == true, "");
                          ^
<source>:30:54: note: function parameter 's' with unknown value cannot be used
in a constant expression
   return test_helper(s, make_index_sequence<strsize(s)>{});
                                                     ^
<source>:29:30: note: declared here
constexpr auto test(const S& s) noexcept-> decltype(auto) {
                             ^


So I will leave it open for a better error message request.

[Bug c++/103478] Possible regression in constexpr processing

[rguenth at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=103478

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
   Last reconfirmed|                            |2021-11-30
             Status|UNCONFIRMED                 |NEW
     Ever confirmed|0                           |1

[Bug c++/103478] Possible regression in constexpr processing

[marxin at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=103478

Martin Liška <marxin at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |jason at gcc dot gnu.org,
                   |                            |marxin at gcc dot gnu.org

--- Comment #3 from Martin Liška <marxin at gcc dot gnu.org> ---
Started with r10-6417-g87fbd5347b338830.

[Bug c++/103478] More explanation for error on constexpr function parameter used in a constant expression

[jason at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=103478

Jason Merrill <jason at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
           Severity|normal                      |enhancement
            Summary|Possible regression in      |More explanation for error
                   |constexpr processing        |on constexpr function
                   |                            |parameter used in a
                   |                            |constant expression