D safety! New Feature?

[Chris Wright via Digitalmars-d]

On Fri, 05 Aug 2016 23:12:06 +0200, ag0aep6g wrote:

> On 08/05/2016 09:39 PM, Mark J Twain wrote:
>> In the case of ImmutableQueue, There is no Enqueue!
>>
>> See, there is a difference between "not callable" and "does not
>> exists".
> 
> Ok, but what cool stuff is enabled by "does not exist" that doesn't work
> (as nicely) with "not callable"? As far as I can tell, there is no
> difference in practice.

Some reflection stuff is a bit inconvenient:

class A {
  int foo() { return 1; }
}

void main() {
  auto a = new immutable(A);
  // This passes:
  static assert(is(typeof(a.foo)));
  // This doesn't:
  static assert(__traits(compiles, () { a.foo; }));
}

__traits(compiles) is mostly an evil hack, but things like this require 
its use.