`in` parameters optimization

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On Wednesday, 25 December 2019 at 01:24:52 UTC, Adnan wrote:
> Does the compiler automatically pass values by reference if 
> possible with `in` parameters in higher level of optimization 
> flags? I would normally use `in ref` but sometimes it's not 
> compatible with different types.

No. "in" is short for "const scope" 
(https://dlang.org/spec/function.html#param-storage). The only 
mechanism D has for expressing "pass this argument by ref if 
possible, and by value otherwise" is "auto ref" 
(https://dlang.org/spec/function.html#auto-ref-functions), which 
requires your function to be templated.