[clang] [Clang][RFC] Resugar attributed type alias (PR #143143)

[Matheus Izvekov via cfe-commits]

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@@ -6998,6 +7007,31 @@ namespace {
         else
           return C.getRValueReferenceType(New);
       }
+      case Elaborated: {
+        auto *ET = cast<ElaboratedType>(Old);
+        return C.getElaboratedType(ET->getKeyword(), ET->getQualifier(),
+                                   wrap(C, ET->getNamedType(), I));
+      }
+      case TypeAlias: {
+        auto *ET = cast<TypedefType>(Old);
+        QualType Underlying = wrap(C, ET->desugar(), I);
+        TypedefNameDecl *Typedef;
+        if (auto *TD = dyn_cast<TypedefDecl>(ET->getDecl())) {
+          Typedef = TypedefDecl::Create(C, TD->getDeclContext(),
+                                        TD->getBeginLoc(), TD->getLocation(),
+                                        TD->getIdentifier(), nullptr);
+          Typedef->setModedTypeSourceInfo(TD->getTypeSourceInfo(), Underlying);
+        } else {
+          auto *Alias = cast<TypeAliasDecl>(ET->getDecl());
+          Typedef = TypedefDecl::Create(
+              C, Alias->getDeclContext(), Alias->getBeginLoc(),
+              Alias->getLocation(), Alias->getIdentifier(), nullptr);
+          Typedef->setModedTypeSourceInfo(Alias->getTypeSourceInfo(),
+                                          Underlying);
+        }
+        Typedef->setPreviousDecl(ET->getDecl());
+        return C.getTypedefType(Typedef, Underlying);
----------------
mizvekov wrote:

Otherwise, passing that second 'Underlying' argument would be unnecessary.

The reason we have that second  parameter is to support a feature called 'divergent typedefs', which was added a couple of years ago in order to make resugaring more efficient.

This is currently used by the getCommonSugaredType infrastructure as well.

But yeah, this allows you to invent a different underlying type for a TypedefType, without having to create a fake TypedefDecl for it.

https://github.com/llvm/llvm-project/pull/143143