[Bug c++/87594] New: constexpr rejects-valid code with range-based for-loop

[mpolacek at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=87594

            Bug ID: 87594
           Summary: constexpr rejects-valid code with range-based for-loop
           Product: gcc
           Version: unknown
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: mpolacek at gcc dot gnu.org
  Target Milestone: ---

Related to PR87567:

// { dg-do compile { target c++14 } }

constexpr bool always_false() { return false; }
int f() { return 1; }

constexpr int
fn1()
{
  struct empty_range {
    constexpr int* begin() { return 0; }
    constexpr int* end() { return 0; }
  } e;
  for (auto x : e)
    f();
  return 0;
}

constexpr int
fn2 ()
{
  int a[] = { 1, 2, 3 };
  for (auto x : a)
    f(); // { dg-error "call to non-.constexpr. function" }
  return 0;
}

constexpr int
fn3 ()
{
  __extension__ int a[] = { };
  for (auto x : a)
    f();
  return 0;
}

template<typename>
constexpr int
fn4 ()
{
  struct empty_range {
    constexpr int* begin() { return 0; }
    constexpr int* end() { return 0; }
  } e;
  constexpr int j = ({ for (auto x : e) f(); 1; });
  return 0;
}

template<typename>
constexpr int
fn5 ()
{
  __extension__ int a[] = { };
  constexpr int j = ({ for (auto x : a) f(); 1; });
  return 0;
}

void
bar ()
{
  constexpr int i1 = fn1 ();
  constexpr int i2 = fn2 ();
  constexpr int i3 = fn3 ();
  constexpr int i4 = fn4<int> ();
  constexpr int i5 = fn5<int> ();
}