[Bug c++/101651] New: constexpr write to simd vector element

[glisse at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=101651

            Bug ID: 101651
           Summary: constexpr write to simd vector element
           Product: gcc
           Version: 12.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: glisse at gcc dot gnu.org
  Target Milestone: ---

(adapted from https://stackoverflow.com/q/68517921/1918193)

#ifdef WORK
 #include <array>
 typedef std::array<char,16> vec;
#else
 typedef char vec __attribute__((vector_size(16)));
#endif
constexpr auto gen () {
    vec ret{};
    for (int i = 0; i < sizeof(vec); ++i) {
        ret[i] = 2;
    }
    return ret;
};
constexpr auto m = gen();


c.cc:9:23:   in 'constexpr' expansion of 'gen()'
c.cc:9:24: error: modification of '(char [16])ret' is not a constant expression
    9 | constexpr auto m = gen();
      |                        ^

However, with -DWORK to use std::array instead of the vector extension, it
compiles just fine, so there shouldn't be any strong obstacle to implement
this.