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

[Michael Park via cfe-commits]

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@@ -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);
+  }
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mpark wrote:

CC @cor3ntin, @mizvekov, @dmpolukhin since they helped look at #121245 and might have some answers around invariants/expectations here.

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