C standard question?

Are the variables on the righthand side of an assignment statement
treated strictly as values?  That is, if in assigning to an "unsigned
int" I shift a "unsigned char" 24 places to the left, can I trust that
the compiler will use temp storage sufficient to hold the "unsigned
int" and NOT result in an overflow (because I shifted an "unsigned
char" 24 places)?

Using gcc I tried the code below:

#include <stdio.h>

int main( int argc, char *argv[] )
{
    unsigned char c[ 4 ] = { 0xff, 0xff, 0xff, 0xff };
    unsigned int ui;

    ui = (c[ 3 ] << 24) | (c[ 2 ] << 16) | (c[ 1 ] << 8) | c[ 0 ];
    fprintf( stderr, "ui = %x\n", ui );
}

and got:

> ui = ffffffff

But validation through compilation is a dangerous thing!

---jski