================
@@ -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
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