[Bug c++/96355] New: [concepts] internal compiler error: in tsubst_pack_expansion, at cp/pt.c:12928

[src at andyf dot de]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=96355

            Bug ID: 96355
           Summary: [concepts] internal compiler error: in
                    tsubst_pack_expansion, at cp/pt.c:12928
           Product: gcc
           Version: 10.2.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: src at andyf dot de
  Target Milestone: ---

The following code (https://godbolt.org/z/edTK48) causes an internal compiler
error:

template<typename... Args>
requires requires
{
   requires sizeof...(Args) > 1;
}
auto add(const Args&... args)
{
   return (... + args);
}

int main()
{
   return add(2, 3, 4);
}


The error is:
<source>: In instantiation of 'auto add(const Args& ...) [with Args = {int,
int, int}]':
<source>:13:22:   required from here
<source>:4:13: internal compiler error: in tsubst_pack_expansion, at
cp/pt.c:12922
    4 |    requires sizeof...(Args) > 1;
      |             ^~~~~~~~~~~~~~~

Please submit a full bug report,
with preprocessed source if appropriate.
See <https://gcc.gnu.org/bugs/> for instructions.



With g++ 10.1 on godbolt there is no internal compiler error, but the compiler
concludes

<source>:4:29: note: nested requirement '(sizeof... (Args) > 1)' is not
satisfied

which is also wrong. However, the behavior seems to have changed with 10.2.0
and exists in trunk (20200727). The code compiles, if I move the requirement
directly into the requires-clause:

template<typename... Args>
requires(sizeof...(Args) > 1)
auto add(const Args&... args)
{
   return (... + args);
}

This version even produces the correct result. I suspect the error is in the
path of the nested requirement.

The failing code compiles with clang 10.0.0 and MSVC 19.24.


Cheers,
   Andreas