-preview=in might break code

[Andrei Alexandrescu]

On 10/4/20 10:08 PM, Walter Bright wrote:
> On 10/3/2020 7:49 AM, Steven Schveighoffer wrote:
>> `in ref` is a reference, and it's OK if we make this not a reference 
>> in practice, because it's const.
> 
> No, it is not. Because a `const ref` can be changed by another mutable 
> reference to the same memory object. This is defined behavior.
> 
> This suggestion turns defined behavior into implementation-defined 
> behavior, meaning it will break existing code written in good faith in 
> unpredictable, unreliable ways.

I will add that C++'s std::min and std::max have had this perennial 
problem that still bites users. Check 
https://en.cppreference.com/w/cpp/algorithm/min:

"Warning
Capturing the result of std::min by reference produces a dangling 
reference if one of the parameters is a temporary and that parameter is 
returned:

int n = 1;
const int& r = std::min(n-1, n+1);
// r is dangling
"

C++ compilers have gotten increasingly adept at detecting and warning 
about such situations, but voluntarily adding a problematic feature to 
the D language to then work on fixing its aftermath doesn't seem a wise 
thing to do.