[ccan] dgraph, tlist, tcon: MSVC error due to array of flexible structs

[Kevin Locke]

Hi all,

Building dgraph using Microsoft Visual C++ produces the following
error:

dgraph.h(25): error C2233: 'edge': arrays of objects containing
zero-size arrays are illegal

The issue is that MSVC supports flexible array members, but does not
support arrays of structs with flexible array members.  My
understanding is that such support is not required by C99 or C11, but
I don't know whether such support is common beyond Clang/GCC.

Is there any interest in supporting such compilers?  If so, I can see
a few potential fixes:

1. Change TCON to use flexible array members only when the compiler
supports arrays of structs with flexible array members.  This is an
easy fix, but wastes space for every TCON/TLIST usage on such
compilers.

2. Have TCON (and TLIST) provide an alternative macro which uses
flexible array members only when supported in arrays, and use that in
dgraph.  It would increase the API and duplicate a bit of code, but
would only waste space when TCON/TLIST are placed in arrays.

2. Replace the 2-element edge array in struct dgraph_node with
separate fields edge_from and edge_to.  This complicates the logic a
bit, since it is no longer symmetric with dgraph_edge.  It would also
be an incompatible API change.

3. Change TLIST to use TCON_WRAP?  I don't understand TLIST/TCON well
enough to know if this is really an option.  Hopefully an expert can
weigh in here.

Thoughts?

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