-preview=in might break code

[Steven Schveighoffer]

On 10/4/20 10:19 AM, Iain Buclaw wrote:
> 3. There is no danger of ref/non-ref mismatches in ABI, because `in` 
> parameters that are inferred `ref` are going to be passed in memory anyway.
> 
> In the following:
> ```
> alias A = void delegate(in long);
> alias B = void delegate(const long);
> ```
> Either `long` always gets passed in memory, or always gets passed in 
> registers, in both cases, they are always going to be covariant.  The 
> same remains true whatever type you replace `long` with.

As the change says, it's up to the back end to decide but is currently 
types over 2 machine word size. So this means on 32-bit systems, 80-bit 
reals would be passed by reference for example.

In practice, I don't know what this means for future or other platform 
compilers.

But as a trivial counter-example, "the same remains true whatever type 
you replace `long` with" isn't correct:

struct S
{
   size_t[3] data;
}

alias A = void delegate(in S);
alias B = void delegate(const S);

static assert(is(A : B)); // fails with -preview=in

One can imagine this being a sticking point where code that builds fine 
on 64-bit systems because of the "lucky chance" that a struct can be 
passed by value is not true on a 32-bit system.

I just found it odd that this was touted as a benefit.

-Steve