russ lyttle <lyttlec@removegmail.com> writes:

> Example from the bookAnalysable Real-Time Systems :
>
> 1  package Environment_Monitor.Ch4_Status is --Protected
> 2    protected type Methane_Agent (Ceiling : Priority)
> 3     with Priority => Ceiling
> 4       is new CH4_Safety
> 5    with
> 6      overriding function Check_Safe return Methane_Status;
> 7      overriding procedure Set_Safety (M : Methane_Status);
> 8    private
> 9       Current_Status : Methane_Status := Motor_Unsafe;
> 10   end Methane_Agent;
> 11 end Environment_Monitor.Ch4_Status;
>
> Gnat complains that Methane_Agent is frozen at line 2, and line 3 is
> trying to change an attribute after being frozen.

The resolution may well be to upgrade your compiler. GNAT GPL 2017 and
FSF GCC 8 are both happy with this compilable demonstrator: