Data types like char and short int take a fewer number of bytes than int. These data types are automatically promoted to int or unsigned int when an operation is performed on them. This is known as integer promotion.
“Whenever any operations like arithmetic, bitwise are performed on lower rank data types such as char and short, automatically these data types will be implicitly promoted to a higher rank data type, signed int or unsigned int, until and unless explicitly typecasted.”
For more details about Integer Promotion, refer the link below.
Satyanarayana Sampangi 