The util package that comes with Modelsim includes an enumerated type forcetype that is defined as:

type forcetype is (default, deposit, drive, freeze);

In VHDL 2008, default is a new keyword. This causes problems in VHDL 2008 projects that use the util package. Still Sigasi Studio allows to use the modelsim_lib.util package in VHDL 2008 projects.

Since the code in modelsim_lib is VHDL 93 code, modelsim_lib should be compiled in VHDL 93 mode. You can do so by right-clicking the modelsim_lib folder in the Common Libraries of your project. Then select Properties > VHDL Version and set the VHDL Version of the modelsim_lib folder to VHDL 1993.

For VHDL 2008 compatibility, package util provides the alias mti_default that can be used in VHDL 2008 code instead of default.

type forcetype is (default, deposit, drive, freeze);
alias mti_default is default [return forcetype];

In VHDL 2008 the util package no longer is needed since the language now supports forcing and releasing of signal values. More information can be found here.