Check if function argument can be handled in CT

[Mike Parker]

On Wednesday, 24 April 2019 at 10:52:58 UTC, Andrey wrote:
> On Wednesday, 24 April 2019 at 08:28:06 UTC, Basile.B wrote:
>> On Wednesday, 24 April 2019 at 07:53:47 UTC, Andrey wrote:
>
> I know about this template. Unfortunally, it doesn't work 
> inside functions.
>> void test(string arg1, string arg2)
>> {
>>     enum isKnown1 = is(typeof((){enum v = arg1;}));
>>     enum isKnown2 = is(typeof((){enum v = arg2;}));
>>     writeln(isKnown1);
>>     writeln(isKnown2);
>> }
>> 
>> void main()
>> {
>>     test("qwerty", "qaz");
>> }
>
> Output:
>> false
>> false
>
> And I don't want to execute function using CTFE. The function 
> should be ran as usual.
>
> Example:
>> char[] test(string arg1, string arg2)
>> {
>>     static if(isCTArg(arg1) && isCTArg(arg2))
>>     {
>>         enum result = "CT: " ~ arg1 ~ " and " ~ arg2; // no 
>> allocations
>>         return result;
>>     }
>>     else
>>     {
>>         import std.format : format;
>> 
>>         auto result = format!"RT: %s and %s"(arg1, arg2); // 
>> at least 1 allocation
>>         return result;
>>     }
>> }
>> 
>> void main()
>> {
>>     // ...
>>     auto val = test(some_CT_or_RT_arg1, some_CT_or_RT_arg2); 
>> // not CTFE.
>>     // ...
>> }

arg1 and arg2 are *never* compile-time arguments. If a function 
is executed at compile time, the function may be passed arguments 
that are compile-time values, but inside the function, the 
arguments are interpreted as if it were running at runtime. That 
means that you can't assign function arguments to enums.