[Saw a response here, but it disappeared before my reply. Posted here anyway in case it clarifies my first paragraph.]

Did you check out the link? https://godbolt.org/g/aaC4B7

My surprise is that none of clang, gcc, or icc give any warning on this with -Wall -Wextra:

  1  void copy_const(const int * const arg) {
  2    int *copy = (int *)arg;
  3    (*copy)++;
  4  }

I agree that the cast on line 2 is legal and requires no warning. My surprise is that there is no warning for the write on line 3, since I think this is undefined behavior according the quoted part of the spec. Isn't this trying "to modify an object defined with a const-qualified type through use of an lvalue with non-const-qualified type"?

I realize that the compiler has no obligation to issue a warning here, and in fact is fully entitled to my first-born as soon as it encounters undefined behavior. Still, it seems like it would be a useful place to offer the user a warning just as it does in the case of direct modification:

  1 void write_const(const int * const arg) {
  2   (*arg)++;
  3 }

clang: "error: read-only variable is not assignable"

gcc: "error: increment of read-only location '*arg'"

icc: "error #137: expression must be a modifiable lvalue"