> conventional, consistent[1], and documented--semantics of only setting errno on error.

From that man page:

> The implementation may also set errno to EINVAL in case no conversion was performed (no digits seen, and 0 returned).

There are error cases where the implementation may set errno to EINVAL. There's not even a guarantee. I did a quick test. errno is only set if you pass an invalid base, or if the string does contain a number but it is out of range. If you pass a string which doesn't even remotely look like a number, errno is not set. You have to check endptr.

  #include <errno.h>
  #include <stdio.h>
  #include <stdlib.h>
  #include <string.h>

  int main(void)
  {
    const char *s = "this is not a number";
    printf("strtol(\"%s\")\n", s);
    errno = 0;
    long a = strtol(s, NULL, 10);
    printf("errno: %d; strerror: %s\n", errno, strerror(errno));
    printf("result: %d\n", a);
    return 0;
  }

Output:

  strtol("this is not a number")
  errno: 0; strerror: Success
  result: 0

https://godbolt.org/z/TbEKss8zM

That's just rubbish design. In what is in my opinion the most common error case, namely that the string doesn't represent a number while you expect that it does, errno is not set.