Here's the definite answer:

  #include <stdio.h>
  
  int main()
  {
   long long unsigned llu;
   if (scanf("%llu", &llu) == EOF) {
    printf("EOF!!\n");
   }
   printf("%llu\n", llu);
  
   unsigned u = llu;
   printf("%u\n", u);
   return 0;
  }

Here's an execution run:

  $ ./a.out 
  9876543210
  9876543210 (llu)
  1286608618 (u)

I tried to compile it as C and C++, with both Clang (14.0.0) and GCC (11.3.0):

  gcc     -Wall -Wextra              # no warning
  g++     -Wall -Wextra              # no warning
  clang   -Wall -Wextra              # no warning
  clang++ -Wall -Wextra              # no warning
  clang   -Wall -Wextra -Weverything # loss of precision warning
  clang++ -Wall -Wextra -Weverything # loss of precision warning

However, the warning goes away if there's an explicit cast:

   unsigned u = (unsigned)llu;

Worse, I still have no warning if I do the narrowing cast then affect it to a wider variable:

   long long unsigned u = (unsigned)llu;
   printf("%llu (u)\n", u);

In C++ I'm warned about the old style cast of course, but using `static_cast` makes the warning go away. And of course the code overflows just like before.

I don't have a good solution to this. Sometimes I do want to lose the precision. Bignum arithmetic for instance. In any case, I'm pretty sure the Keccak team's compiler did not issue any warning. Sorry if I implied otherwise.