[Issue 12492] New: [AA] Clarify what types can be used to get associative array key value

[d-bugmail at puremagic.com]

https://d.puremagic.com/issues/show_bug.cgi?id=12492

           Summary: [AA] Clarify what types can be used to get associative
                    array key value
           Product: D
           Version: D2
          Platform: All
        OS/Version: All
            Status: NEW
          Severity: normal
          Priority: P2
         Component: DMD
        AssignedTo: nobody at puremagic.com
        ReportedBy: verylonglogin.reg at gmail.com


--- Comment #0 from Denis Shelomovskij <verylonglogin.reg at gmail.com> 2014-03-30 13:02:34 MSK ---
Currently there is no clear rules about types which can be used to get
associative array key value, e.g. one may expect to be able to get
`int[string]` AA value using `char[]` and vice-versa.

In case of setting value it's simple, a key expression type should be
implicitly convertible to associative array key type (i.e. we need just fix
Issue 2954). The case of getting AA value is more complicated because all we
need is:
1. Ability to compare values of these types.
2. Equality of hash sums of equal values of these types.

So at least this mean `const`/`immutable` type qualifiers can be ignored iff we
always use same hash algorithms regardless of type qualifiers.

It's a bug, not an enhancement because current compiler behaviour is
inconsistent, e.g. it allows getting `int[Object[]]` AA value using `immutable
Object[]` but disallows getting `int[Object]` AA value using `immutable Object`
and even `const Object`.


Also see Issue 12491 about the idea to disallow any non-`immutable` types as
associative array keys.

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