Why is the TREE_PURPOSE of a CONSTRUCTOR_ELT list for ARRAY_TYPE NULL?

[Richard Kenner]

    Except in the constructors i'm dealing with (C++ vtables, represented as
    arrays), all the TREE_PURPOSES in the constructor list are NULL.
    Does that mean that the corresponding index is the last index + 1?

    In any case, is this a documentation bug, or a C++ FE bug (IE should i
    expect tree_purpose to always be there, and it's just not getting set in
    the case of the C vtables, or do the docs need updating)?

It's a C++ FE bug. Before tree-ssa, TREE_PURPOSE could be (and often was)
NULL, but it can't be now.