relates to:

• SR-1275 Implement SE-0025 - Scoped Access Level

Issue Description:

To implement Swift 3's private I introduced a new concept called "access scope", which is used to check if it's okay to use a particular declaration. This is described pretty well in the commit message for 758cf64; the primary entry point is ValueDecl::getFormalAccessScope.

Currently, this function just returns a DeclContext like any other. There are two problems with this: (1) this isn't just any DeclContext, and it's often passed as an argument next to other DeclContexts (usually the use site), and (2) it erases the distinction between private and fileprivate for top-level declarations, which leads to diagnostics claiming something is fileprivate even though the user has written private.

Instead, we should create a new type AccessScope which just wraps a DeclContext, along with a bit saying whether a top-level DeclContext should be treated as private rather than fileprivate.

class AccessScope {
  llvm::PointerIntPair<const DeclContext *, 1, bool> value;
public:
  // useful constructors
  bool isPrivate() const { /*…*/ }
  const DeclContext *getDeclContext() const { /*…*/ }
  // more useful methods
};

The steps to do this are roughly:

1. Make a new AccessScope type that just wraps a plain DeclContext.
2. Propagate it into all uses of ValueDecl::getFormalAccessScope, transitively.
3. Turn all operations that combine access scopes into methods on AccessScope.
4. Add the "private" flag, using PointerIntPair as shown above to keep the overhead low.
5. Update or replace accessibilityFromScopeForDiagnostics in TypeCheckDecl.cpp to check the "private" flag.

I've tagged this as "StarterBug" because even though it involves a bit of spelunking through the codebase, each of these steps should be fairly obvious (i.e. there's not much design work), and you still get diagnostic improvements out at the end. The separate steps can also be reviewed separately, following Swift's incremental development model.

Please feel free to ask me for help on swift-dev!