Re: [PATCHv3 1/6] Add missing includes and forward declares

On Tue, Aug 14, 2018 at 11:51 PM Elijah Newren [off-list ref] wrote:

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enums are of unknown size, so forward declarations don't work for
them.  See bb/pedantic for some examples.

structs are also of unknown size; the size is irrelevant when the
function signature merely uses a pointer to the struct or enum.  The
enum forward declaration fixes a compilation bug.

My rationale may miss the point but the standard and some real compilers
don't like this, unfortunately.

For structs, having an incomplete type is fine, but for enums we need
the full definition.  E.g. C99 sayeth (in section 6.7.2.3 "tags")

        A type specifier of the form

                enum identifier

        without an enumerator list shall only appear after the type it
        specifies is complete.

What about a type specifier of the form
  enum identifier *
?  Can that kind of type specifier appear before the full definition
of the enum?  (Or, alternatively, if the standard doesn't say, are
there any compilers that have a problem with that?)

If so, we can include cache.h instead.  We'll probably also have to
fix up packfile.h for the exact same issue (even the same enum name)
if that's the case.

Digging a little further this morning, apparently C++ has defined a
forward declaration of an enum to either be useless (because it was
already defined), require an explicit size specifier, or be a
compilation error.

That seemed stupid to me, but a little more digging turned up
http://c-faq.com/null/machexamp.html , which states that sizeof(char*)
!= sizeof(int*) on some platforms.  That was a big surprise to me.
Since an enum could be a char or int (or long or...), knowing the size
of the enum thus is important to knowing the size of a pointer to an
enum, so we actually do need the full enum definition (or a C++ style
explicit size specifier).

What a crazy world.

So, I'll go with the inclusion of cache.h and also fix up packfile.h
the same way.  Thanks for pointing this out, Jonathan.