Demotion represents a potential problem with truncation or unpredictable results often occurring. How do you ft an integer value of 456 into a character value? How do you ft the floating-point value of 45656.453 into an integer value? Most compilers give a warning if it detects demotion happening. A compiler warning does not stop the compilation process. It does warn the programmer to check to see if the demotion is reasonable.
If I calculate the number of cans of soup to buy based on the number of people I am serving (say 8) and the servings per can (say 2.3), I would need 18.4 cans. I might want to demote the 18.4 into an integer. It would truncate the 18.4 into 18 and because the value 18 is within the domain of an integer data type, it should demote with the truncation side effect.
If I tried demoting a double that contained the number of stars in the Milky Way galaxy into an integer, I might have a get an unpredictable result (assuming the number of stars is larger than allowable values within the integer domain).