[Bug c++/94074] New: bogus modifying a const object error with const COMPONENT_REF

[mpolacek at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=94074

            Bug ID: 94074
           Summary: bogus modifying a const object error with const
                    COMPONENT_REF
           Product: gcc
           Version: 10.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: mpolacek at gcc dot gnu.org
  Target Milestone: ---

typedef decltype (sizeof (0)) size_t;

template <typename E, size_t N>
struct array
{
  constexpr const E &operator[](size_t n) const noexcept { return elems[n]; }
  E elems[N];
};

template <typename T>
struct S {
  using U = array<T, 4>;
  U m;
  constexpr S(int) : m{}
  {
    const_cast<int &>(const_cast<const U &>(m)[0]) = 42;
  }
};

constexpr S<int> p = { 10 };

$ ./cc1plus -quiet q.C
q.C:20:27:   in ‘constexpr’ expansion of ‘S<int>(10)’
q.C:16:52: error: modifying a const object ‘(int&)(&(&(const
U&)(&((S<int>*)this)->S<int>::m))->array<int, 4>::operator[](0))’ is not
allowed in a constant expression
   16 |     const_cast<int &>(const_cast<const U &>(m)[0]) = 42;
      |     ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~^~~~
q.C:20:27: note: originally declared ‘const’ here
   20 | constexpr S<int> p = { 10 };
      |                           ^

ISTM we shouldn't error here.