Checking if CTFE is used?

[Adam D. Ruppe]

On Tuesday, 18 December 2018 at 14:23:38 UTC, berni wrote:
>> import std.stdio;
>>
>> class A
>> {
>>     static immutable int[4] clue;
>> 
>>     static this()
>>     {
>>         if(__ctfe) assert(0, "Yep, CTFE used");
>>         foreach (i;0..4) clue[i] = i;
>>     }
>> }

CTFE is used if and only if it MUST be used by context. That's a 
runtime function, so no ctfe.

Do something like:

int[4] generate() {
    int[4] tmp;
    foreach(i; 0..4) tmp[i] = i;
    return tmp;
}


static immutable int[4] clue = generate();



Since it is an immediate static immutable assign, ctfe MUST be 
used, and thus will be used. With your code, you used a runtime 
constructor on all runtime variables, so it did it at program 
startup.

Generally:

static immutable something = something; // must be ctfe
enum something = something; // makes a ctfe literal (but not 
necessarily static storage)


so those patterns force ctfe. While

static immutable something;
something = something; // NOT ctfe because it is a separate 
statement!