When parsing a VHDL design, XST issues the following error message:
"ERROR:HDLParsers:800 - <file>.vhd Line xx. Type of <inst> is incompatible with type of <type>."
This message might occur for a variety of mismatches. The values of "inst" and "type" listed in the error message are the key to resolving the problem.
The most common cause of this error is a mismatch between type of assignment and operator result. For example, if an operation returns an "std_logic" value, and the assignment is to a signal of type "std_logic_vector" (or integer, real, etc.), this error occurs.
This error will also happen when using different IEEE libraries in different files of the project which has the same definitions.