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The problem is what to do with the "when others" statements...
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Do not use latches, whether intentionally or unintentionally. See "Unintentional Latch Generation" in the Quartus handbook, Volume 1, Section II, Chapter 6.
The "State Machines" section of that chapter says this about "when others":
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Many synthesis tools (including Quartus II integrated synthesis) have an option to implement a safe state machine. The software inserts extra logic to detect an illegal state and force the state machine’s transition to the reset state.
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