[patch/rfc] An opt-in warning for functions whose availability(introduced) is newer than the deployment target

[Douglas Gregor]

Hi Nico,

> On Jan 8, 2015, at 6:14 PM, Nico Weber <thakis at chromium.org> wrote:
> 
> Hi,
> 
> the Mac OS X and iOS SDKs add new functions in new releases. Apple recommends using the newest SDK and setting the deployment target to whatever old OS version one wants to support, and only calling new functions after checking that they are available at runtime.
> 
> In practice, we (Chromium) get this wrong. Others who support old OS X versions get this wrong too. Hence, we (Chromium) use a very old SDK and then manually declare new functions when we want to call them – this reduces the chance of us forgetting if they are available at runtime considerably, in practice. But using an old SDK has its problems – sometimes the frameworks check which SDK an app was linked against and only then activate bug fixes, and newer Xcodes don't ship include old SDKs.

That’s an interesting approach to handling the availability problem; I hadn’t heard of it before, but I see the logic there.

> Ideally, we could use a new SDK but get a warning when we use a new API without a manual redeclaration – this protects us against new APIs the same way using an old SDK does without the drawbacks that this brings.
> 
> The attached patch is a sketch how such a warning might work. How repulsive is this idea? Are there other approaches to this problem? If the basic idea is ok:

This is a drastically different approach than I’d imagined. Whenever I’ve thought about this problem, I’ve always come back to some form of dataflow analysis that checks whether uses of “not-yet-introduced” API is used in a sane way: is it dominated by some check that implies the availability, e.g., a -respondsToSelector: check on a method with at least that availability, or checking whether “[NSFoo class]” is non-null when the class has availability. I suspect that’s the idea behind Deploymate (http://www.deploymateapp.com <http://www.deploymateapp.com/>), although I’ve never used it, and it has the benefit that it should make idiomatic code (that does the right checking) just work.

It’s also a heck of a lot more work to implement than the approach you’re using.

> Any comments on the implementation?

The implementation generally looks fine. One minor comment:

+    case AR_NotYetIntroduced: {
+      // don't do this for enums, they can't be redeclared.
+      if (isa<EnumConstantDecl>(D) || isa<EnumDecl>(D))
+        break;
+      bool FoundRedecl = false;
+      for (Decl *Redecl = D->getMostRecentDecl(); Redecl && !FoundRedecl;
+           Redecl = Redecl->getPreviousDecl()) {
+        if (Redecl->getAttr<AvailabilityAttr>()->isInherited())
+          FoundRedecl = true;
+      }
+      if (!FoundRedecl)
+        S.EmitAvailabilityWarning(Sema::AD_Partial, D, Message, Loc,
+                                  UnknownObjCClass, ObjCPDecl,
+                                  ObjCPropertyAccess);
+      break;
+    }

Generally speaking, name lookup will always find the most recent declaration, so you might be able to skip the D->getMostRecentDecl() bit entirely and just check that the availability attribute was inherited.

	- Doug


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