D's treatment of values versus side-effect free nullary functions

[Walter Bright]

Andrei Alexandrescu wrote:
> Walter Bright wrote:
>> Don wrote:
>>> While running the semantic on each function body, the compiler could 
>>> fairly easily check to see if the function is CTFEable. (The main 
>>> complication is that it has to guess about how many iterations are 
>>> performed in loops). Then, when a CTFEable function is called with 
>>> compile-time constants as arguments, it could run CTFE on it, even if 
>>> it is not mandatory.
>>
>> I think this is the halting problem, and is insoluble.
> 
> On what basis?

Trying to determine by static analysis if a function would evaluate within a 
"reasonable" amount of time, or even if it would finish at all.

So let's suppose the compiler just attempts CTFE on those functions anyway, with 
a timeout if they take too long. Suddenly we've just thrown all the compiler 
performance out the window.