[Bug c++/68888] New: No Warning when converting an array of a subclass to its parent

[gcc-bugs at bluematt dot me]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=68888

            Bug ID: 68888
           Summary: No Warning when converting an array of a subclass to
                    its parent
           Product: gcc
           Version: 5.3.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: gcc-bugs at bluematt dot me
  Target Milestone: ---

Without bringing up the deadly "warn when using arrays in function
declarations" quagmire, if you are converting an array of one type to that of
another due to pointer decay, there should probably at least be a huge warning.

eg the following prints "1 2" to the surprise of many unthinking programmers.

#include <stdio.h>

class A {
public:
        int a;
};

class B : public A {
public:
        int b;
};

void go(A b[2]) {
        fprintf(stderr, "%d %d\n", b[0].a, b[1].a);
}

int main() {
        B bs[2];
        bs[0].a = 1;
        bs[0].b = 2;
        bs[1].a = 3;
        bs[1].b = 4;
        go(bs);
}