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There are existing compiler implementations that allow const-qualified values objects to be modified without generating a warning message.

Avoid casting away const qualification because doing so makes it easier to modify const-qualified values objects without issuing diagnostics.  See EXP05-C. Do not cast away a const qualification for more details.

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The following well-formed, but noncompliant, code example specified in subclause 6.5.16.1 of the C Standard allows a constant value object to be modified:

const char **cpp;
char *p;
const char c = 'A';

void func(void) {
  cpp = &p; /* Constraint violation */
  *cpp = &c; /* Valid */
  *p = 0; /* Valid */
}

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