On 4/10/2012 5:03 PM, David Nadlinger wrote:
On 10 Apr 2012, at 19:09, Walter Bright wrote:
Includes fixes for all […]
I'll leave you with the last word on this, but I'm still not quite happy with
the idea that the result of 7868 [1] is to just accept that an expression
silently yields a different result if it is somehow evaluated from a static if
condition. The (semi-)regression took me quite some time to track down due to
its elusiveness and you some hours to make sense of, and I'm afraid that it
will hit other users as well (who probably have no idea about how to debug the
frontend).
I am not quite sure about the best way to fix this particular problem – from
trying to figure out a way to get the Thrift code working with current Git
master it seems that DMD more or less no longer allows adding something to a
type based on its current members. It is clear that the semantics of this must
be carefully defined, but it is useful for certain »declarative« kinds of
meta-programming (especially due to the lack of something like ADL allowing
you to »associate« code with a type in another way), it worked in previous DMD
releases, and now fails in confusing ways. Maybe we finally need to sit down
and formalize our forward reference model (perhaps along the lines of the
proposal that came up some time ago – was it by Don? –, for doing semantic
analysis one layer of conditionals at a time).
The underlying problem is really that as a developer doing
metaprogramming-heavy stuff, you have no idea what actually is supposed to
work, and after you found out the hard way by testing what DMD accepts, you
still don't know whether it will continue to work in future releases. And this
is a much bigger problem than e.g. breaking standard library changes, because
you can easily work around the latter.
I mostly agree with you, and apologize for the trouble it caused you. The fact
that it worked at all before was a fluke, caused by incomplete checking by the
compiler, and that incomplete checking caused numerous other problems.
Allowing forward references in general is a very tricky problem in the situation
of circular definitions. The strategy that dmd originally followed was to do
things in lexical order, and dang the consequences. The more advanced strategy
dmd has been migrating to is to allow circular references as long as there is
enough information about the partially analyzed type to break the cycle. For
example, sometimes only the size of a struct is required. Therefore, dmd
requires only that the members of the struct that contribute to its size are
analyzed - the other members need not be.
In your case, you had a static if turning on and adding members by checking a
condition that must check all the members. This cannot possibly work. I spent a
lot of type trying to figure out a way that the cycle could be legitimately
broken, but could not.
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