[Bug libstdc++/84886] New: Add static checking to library components

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=84886

            Bug ID: 84886
           Summary: Add static checking to library components
           Product: gcc
           Version: 8.0
            Status: UNCONFIRMED
          Keywords: diagnostic
          Severity: normal
          Priority: P3
         Component: libstdc++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: redi at gcc dot gnu.org
  Target Milestone: ---

There are several places where we could diagnose undefined behaviour:

#include <array>
const std::array<int, 0> a0{};
int i0 = a0[0];                  // undefined
const std::array<int, 1> a1{};
int i1 = a1[1];                  // undefined


#include <string_view>
const std::string_view s;
char c0 = s[0];                  // undefined
char c1 = s.front();             // undefined
// not even in constant expressions:
constexpr std::string_view s1("a");
constexpr char c2 = s1[1];       // undefined

#include <atomic>
int main() {
  std::atomic<int> a{};
  a.load(std::memory_order_release);  // undefined
}

These can be turned into compile-time errors:
https://gcc.gnu.org/ml/libstdc++/2018-03/msg00031.html
https://gcc.gnu.org/ml/libstdc++/2018-03/msg00032.html

For std::array<N, 0> we need to ensure explicit instantiation works, without
operator[] causing errors.