How to know if opSlice is defined for a type (including built-in types)?

[Rikki Cattermole via Digitalmars-d-learn]

On 20/01/16 11:41 PM, chardetm wrote:
> Hi everyone,
>
> I was wondering if it was possible to know at compile time if opSlice is
> defined for a type, including built-in types like "string". Here is the
> code I have:
>
>
> import std.stdio;
>
> struct Test {
>      Test opSlice(size_t i, size_t j) {
>          return this; // We don't care what it returns
>      }
> }
>
> void main () {
>      string s = "Hello";
>      auto s2 = s[0..2];
>      static if (is(typeof(&string.opSlice))) writeln("Ok string");
>
>      Test t;
>      auto t2 = t[0..2];
>      static if (is(typeof(&Test.opSlice))) writeln("Ok Test");
> }
>
> Output:
> Ok Test
>
>
> How to make it work for "string" too (if possible)? And is there a way
> to differentiate "opSlice()" from "opSlice(size_t, size_t)" (or
> "opSlice(T, T)" for any T)?
>
> Thank you in advance!

template CanSlice(T) {
	enum CanSlice = __traits(compiles, {T t; auto v = t[0 .. 1];}) || 
__traits(compiles, {T t; auto v = t.opSlice();});
}

Should work.