Why can't I pack two independent 18x18 multipliers into single Arria® V DSP block?

Description

Under certain conditions, the Quartus® II software may not be able to pack two independent 18x18 multipliers into the same Arria® V DSP Block.  

Resolution

To work around this issue, add the following line into your quartus.ini file under the project directory:

fsac_ff_enable_mac_scan_chain_packing=OFF

If you do not already have a quartus.ini file, use the text editor and create this file in your project directory, then add the above line. 

This problem is fixed in Quartus® 13.1.