[Bug c++/70228] New: insufficient detail in diagnostics for a constexpr out of bounds array subscript

[msebor at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=70228

            Bug ID: 70228
           Summary: insufficient detail in diagnostics for a constexpr out
                    of bounds array subscript
           Product: gcc
           Version: 6.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: msebor at gcc dot gnu.org
  Target Milestone: ---

Diagnostics issued for out of bounds array references in non-trivial constexpr
don't provide sufficient detail to quickly root cause the problem.  For
example, the diagnostic printed for the program below makes determining the
value that exceeded the bounds of the array and what those bounds are more
difficult than it should be.  Since the index value and the bounds are known at
translation time, they both should be mentioned in the text of the diagnostic.

$ cat v.c && /home/msebor/build/gcc-trunk-git/gcc/xgcc
-B/home/msebor/build/gcc-trunk-git/gcc -S -Wall -Wextra -Wpedantic -o/dev/null
-xc++ v.c
constexpr int a[] = { 1, 2, 3 };

constexpr int f (int i)
{
    int j = i + 1;
    return a[j];
}

constexpr int g (int i)
{
    int j = i + 1;
    return f (j);
}

constexpr int i = g (0);
constexpr int x = g (i);
v.c:16:21:   in constexpr expansion of ‘g(3)’
v.c:12:14:   in constexpr expansion of ‘f(j)’
v.c:16:23: error: array subscript out of bound
 constexpr int x = g (i);
                       ^

For an example of what a more helpful diagnostic might look like consider the
output printed by Clang for the same program:

v.c:16:15: error: constexpr variable 'x' must be initialized by a constant
      expression
constexpr int x = g (i);
              ^   ~~~~~
v.c:6:12: note: cannot refer to element 5 of array of 3 elements in a constant
      expression
    return a[j];
           ^
v.c:12:12: note: in call to 'f(4)'
    return f (j);
           ^
v.c:16:19: note: in call to 'g(3)'
constexpr int x = g (i);
                  ^
1 error generated.