[clang] [lldb] [C++20][Modules] Do not update the declaration generation number if the redeclaration chain completion was delayed. (PR #129982)

[Michael Park via cfe-commits]

================
@@ -9180,6 +9180,12 @@ bool Sema::hasAcceptableDefinition(NamedDecl *D, NamedDecl **Suggested,
   if (!getLangOpts().Modules && !getLangOpts().ModulesLocalVisibility)
     return true;
 
+  // The external source may have additional definitions of this entity that are
+  // visible, so complete the redeclaration chain now.
+  if (auto *Source = Context.getExternalSource()) {
+    Source->CompleteRedeclChain(D);
+  }
----------------
mpark wrote:

Hm, okay... so the `DC->lookup(Name)` have different behaviors because of [`Source->FindExternalVisibleDeclsByName(...)`](https://github.com/llvm/llvm-project/blob/release/20.x/clang/lib/AST/DeclBase.cpp#L1913). In `ASTReader::FindExternalVisibleDeclsByName`, we get to [here](https://github.com/llvm/llvm-project/blob/c6d95c441a29a45782ff72d6cb82839b86fd0e4a/clang/lib/Serialization/ASTReader.cpp#L8494-L8501) looking through `Lookups` data structure. In the first call, `Lookups[DC].Table[Name]` has 1 entry (just a decl), whereas in the second call it has 2 entries (the decl and a defn).

In summary, we have `LazyGenerationalUpdatePtr::get` calling `CompleteRedeclChain` where `Lookups` isn't populated with the definition, and updates the generation number. The second time `LazyGenerationUpdatePtr::get` is called, `Lookups` **is** populated with the definition, but the generation number matches, and so it doesn't call `CompleteRedeclChain` even though it would yield a different result.

https://github.com/llvm/llvm-project/pull/129982