Thoughts about deprecation

[Stewart Gordon]

Deprecation is a nice feature.  There doesn't seem to be any doubt about it.

But it's by no means perfect, compiler bugs aside.  I have identified these ideals of 
deprecation:

(a) Code that compiles without -d compiles and behaves the same with -d.

(b) Code that compiles without -d compiles and behaves the same if all entities declared 
as deprecated are removed from the code.

(c) Code that compiles with or without -d compiles and behaves the same if all deprecated 
entities are de-deprecated.

(d) Code that compiles without -d either compiles and behaves the same, or fails to 
compile at all, if any entities are newly deprecated (i.e. does not silently change its 
behaviour).

(e) Validity checking through compile-time reflection is always consistent with whether 
the compiler actually accepts the code.

(OK, so the switch used to enable deprecated entities might vary between brands of 
compiler, but I've used "-d" here for simplicity.)


But these ideals cannot all be satisfied at once.

The main reason for this seems to be compile-time reflection.  My experiment (under DMD 
1.071 and 2.057) shows that an IsExpression evaluates to false if the compiler would 
reject it because of deprecation.  This satisfies ideals (b) and (e), but violates (a), 
(c) and (d).

If we changed it to evaluate to true even if compiling without -d, then it would satisfy 
(a), (c) and (d), but violate (b) and (e).  Moreover, when/if we get -di
http://d.puremagic.com/issues/show_bug.cgi?id=7041
what would it do if we have deprecation-dependent behaviour?

But deprecated entities aren't always treated as though they're not there if compiling 
without -d.  For example, deprecated functions take part in overload resolution.  This is 
necessary in order to satisfy (a), (c) and (d).  Though it doesn't seem to make any 
difference to (b) or (e).


Maybe DDB is useful at times.  The use case that comes to mind is suppressing the handling 
of a deprecated property as part of a non-deprecated method.  But still, is accidentally 
programming DDB in something that needs to be guarded against?  Or do we just accept DDB 
as a natural, unpreventable consequence of reflection?

I'm not sure whether all ideals would be satisfied in the absence of CTR, or if there are 
other aspects of D that prevent it from being so.

I suppose the overall point is: Is the current compiler behaviour the best that can be 
done?  Which ideals of deprecation do we really need to follow, and which can we do without?


Stewart.