If your project contains VHDL 2008 files, you also have to use the 2008 version of the STD and IEEE libraries.

  1. Make sure your project is set to VHDL 2008 :
    Right click your project and select Properties > VHDL version
  2. Right click the Common Libraries folder of your Sigasi project and select Set Library > Reset Common Libraries

Sigasi Studio 3.6 will offer a quick-fix to make this easier.