[Issue 7753] New: Support opIndexCreate as part of index operator overloading in user-defined types

[d-bugmail at puremagic.com]

http://d.puremagic.com/issues/show_bug.cgi?id=7753

           Summary: Support opIndexCreate as part of index operator
                    overloading in user-defined types
           Product: D
           Version: D2
          Platform: All
        OS/Version: All
            Status: NEW
          Severity: enhancement
          Priority: P2
         Component: DMD
        AssignedTo: nobody at puremagic.com
        ReportedBy: hsteoh at quickfur.ath.cx


--- Comment #0 from hsteoh at quickfur.ath.cx 2012-03-22 21:18:10 PDT ---
Currently, a nested indexing expression such as:

    a[b][c][d] = 0;

for a user-defined type that overloads opIndex* gets translated into:

    a.opIndex(b).opIndex(c).opIndexAssign(0,d);

However, if a[b][c] do not yet exist, this will fail. This works correctly for
built-in associative arrays, because the expressions get translated into a
series of calls to _aaGetX(), which creates new entries if they don't already
exist. But currently, there is no way for a user-defined type to accomplish the
same thing.

Suggested fix: if the expression as a whole is being assigned to with an
assignment operator, then the upper-level indexing calls should be translated
into opIndexCreate() instead of just opIndex():

    a[b][c][d] = 0;

becomes:

    a.opIndexCreate(b).opIndexCreate(c).opIndexAssign(0,d);

If opIndexCreate is not defined, then replace it with opIndex (for backward
compatibility). The semantics of opIndexCreate(k) is to return the entry
indexed by k if it exists, and if it doesn't, create a new entry with key k and
the .init value of the value type, and return the new entry.

Preferably, this will apply to any expression that ends with a call to
opIndexAssign, opIndexUnary, and opIndexOpAssign. But at the very least, this
needs to work when the expression ends with opIndexAssign.

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