what's the semantics of 'varX is varY', in particular `strX is strY`?

[Paul Backus]

On Wednesday, 5 August 2020 at 17:29:09 UTC, mw wrote:
> I've thought it's compare by reference, as in e.g. `assert(obj 
> !is null)`
>
> but
>
> ```
>   string strX = "2020-07-29";
>   string strY = "2020-07-29";
>   string strZ = "2020-07-30";
>   assert(strX !is strY);  // assertion failed
>   assert(strX !is strZ);  // assertion pass
> ```
>
> so here `is` means compare strings by their contents? where is 
> the exact definition of the  semantics of 'varX is varY'?

It's documented under the name "Identity Expression":

https://dlang.org/spec/expression.html#IdentityExpression

You're correct that it's comparing by reference. The reason strX 
and strY are equal is that the compiler noticed you wrote two 
identical string literals, so it combined them in the binary. You 
can tell this is true because `assert(strX.ptr == strY.ptr)` 
passes.