[Issue 18604] New: in parameter storage class should be deprecated

[d-bugmail at puremagic.com]

https://issues.dlang.org/show_bug.cgi?id=18604

          Issue ID: 18604
           Summary: in parameter storage class should be deprecated
           Product: D
           Version: D2
          Hardware: All
                OS: All
            Status: NEW
          Severity: enhancement
          Priority: P1
         Component: dmd
          Assignee: nobody at puremagic.com
          Reporter: greensunny12 at gmail.com

`in` currently just means `const` even though it was supposed to mean `scope
const`. This makes it redundant and confusing and should be deprecated. Maybe
after it has been deprecated and been an error for one version, it can be
re-introduced with the proper meaning, but for now it should definitely trigger
a deprecation warning.


> Walter: I don't think we can fix in. It may break an astonishing amount of code, since it was never enforced.

See also:

https://github.com/dlang/druntime/pull/2139#issuecomment-372455730
https://dlang.org/spec/function.html#parameters

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