[Issue 19239] New: Fix unsafe casting away of const in hashOf(const Object)

[d-bugmail at puremagic.com]

https://issues.dlang.org/show_bug.cgi?id=19239

          Issue ID: 19239
           Summary: Fix unsafe casting away of const in hashOf(const
                    Object)
           Product: D
           Version: D2
          Hardware: All
                OS: All
            Status: NEW
          Severity: normal
          Priority: P1
         Component: druntime
          Assignee: nobody at puremagic.com
          Reporter: n8sh.secondary at hotmail.com

`Object.toHash` is not const. Since a struct's `toHash` needs to be const,
usually a struct will just cast away const-ness of an Object and hope that this
violation of the type system doesn't cause a catastrophe. This happens to be
okay for objects that don't override `toHash` because the base implementation
is based on the object's address and is logically const even though it doesn't
have the `const` attribute. We can make this sound by performing a runtime
check to see if it uses the address-based Object.toHash. (It would be even
better if we could perform a runtime check to see if its overridden with a
const function, but I don't believe that is a way to do this.) Doing this will
let us legitimately call `const(Object).toHash()` in `@trusted` code.

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