[libcxx-commits] [PATCH] D140584: [Clang] Refactor "Designators" into a unified implementation [NFC]

[Richard Smith - zygoloid via Phabricator via libcxx-commits]

rsmith added inline comments.


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Comment at: clang/include/clang/AST/Designator.h:88
+  /// An array designator, e.g., "[42] = 0" and "[42 ... 50] = 1".
+  template <typename Ty> struct ArrayDesignatorInfo {
+    /// Location of the first and last index expression within the designated
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void wrote:
> void wrote:
> > rsmith wrote:
> > > Can we move the templating out from here to the whole `Designator` and `Designation` classes? It shouldn't be possible to mix the two kinds in the same `Designation`.
> > Grr...My previous comment was eaten.
> > 
> > I'll give it a shot.
> > 
> > However, I'm a bit surprised at how designators are handled by Clang. I expected that a `Designation` would be an `Expr` with the `Designator`s being L-values (e.g. `MemberExpr`s / `ArraySubscriptExpr`s), but instead the `Designation` exists just long enough to be turned into an explicit initialization list. Is there a reason to do it that way instead of using expressions?
> So it looks like moving the template outside of the class won't work. The ability to switch between `Expr` and `unsigned` while retaining the same overall type is hardwired into things like the `ASTImporter`.
> 
> This is kind of a massive mess. Maybe we shouldn't even allow them to use both `Expr` and `unsigned` but instead require them to use one or the other? Maybe we could require `unsigned` with the understanding that the `Expr` can be converted into a constant?
I'm not understanding something. Currently the `ASTImporter` only deals with `DesignatedInitExpr::Designator`s , which only ever store integer indexes.

Basically, today, we have two different classes:
- A class that's specific to `DesignatedInitExpr`, and tracks array index expressions by storing the index of the expression within the `DesignatedInitExpr`'s list of children; this is also what `ASTImporter` can import, because it's the one that's used in the AST's representation.
- A class that's specific to `Sema`'s processing that tracks array index expressions as `Expr*` instead.

You want to refactor them to share code, which makes sense, because they are basically the same other than how they refer to expressions. (Not quite: `DesignatedInitExpr` can apparently refer to a field either as an `IdentifierInfo*` or as a `FieldDecl*`, whereas the `Sema` version always uses the `IdentifierInfo*` representation.)

Each current user of one of these two classes uses only one of the two, which means they're either exclusively using integers to refer to expressions or exclusively using `Expr*`. So it seems to me that you should be able to update each user to use either `Designator<unsigned>` or `Designator<Expr*>`, depending on which class they used before.

What am I missing?


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https://reviews.llvm.org/D140584