I wish all qualifiers were revisited with an eye for simplification

Andrei Alexandrescu SeeWebsiteForEmail at erdani.org
Mon Aug 17 11:23:04 UTC 2020

On 8/14/20 9:35 PM, Timon Gehr wrote:
> On 15.08.20 03:29, Timon Gehr wrote:
>>> Simple questions should have simple answers.
>> If the above implementation of isCopyable is not the one you want, 
>> something else is off.
> Oh, actually I misspoke, didn't notice that you changed my suggestion 
> from the github issue from __traits(compiles, ...) to is(typeof(...)).
> Use __traits(compiles, ...), not is(typeof(...)). typeof assigns types 
> to expressions that would not otherwise compile, even lambdas:
> void main(){
>      int x;
>      static void foo(){
>          pragma(msg, typeof(()=>x));             // int delegate() pure 
> nothrow @nogc @safe
>          pragma(msg, __traits(compiles, ()=>x)); // false
>      }
> }
> There is this persistent myth that __traits(compiles, ...) is the same 
> as is(typeof(...)) this is not the case and IIRC I have used it to 
> demonstrate that most template constraints in Phobos don't work correctly.

This is a problem as __traits should be a lower-level, seldom-used 
resort. Are the distinctions between the two documented somewhere?

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