From 5485c7021aca771f782c2fc1bc5e913dbed0ddc8 Mon Sep 17 00:00:00 2001
From: Matheus Izvekov <mizvekov@gmail.com>
Date: Tue, 19 Aug 2025 14:32:08 -0300
Subject: [PATCH] [clang] fix redecl chain assumption when checking linkage
 consistency (#153996)

In C++, it can be assumed the same linkage will be computed for all
redeclarations of an entity, and we have assertions to check this.

However, the linkage for a declaration can be requested in the middle of
deserealization, and at this point the redecl chain is not well formed,
as computation of the most recent declaration is deferred.

This patch makes that assertion work even in such conditions.

This fixes a regression introduced in
https://github.com/llvm/llvm-project/pull/147835, which was never
released, so there are no release notes for this.

Fixes #153933
---
 clang/lib/AST/Decl.cpp          | 17 ++++++++++-------
 clang/test/Modules/GH153933.cpp | 23 +++++++++++++++++++++++
 2 files changed, 33 insertions(+), 7 deletions(-)
 create mode 100644 clang/test/Modules/GH153933.cpp

diff --git a/clang/lib/AST/Decl.cpp b/clang/lib/AST/Decl.cpp
index 12fe5516883e..4507f415ce60 100644
--- a/clang/lib/AST/Decl.cpp
+++ b/clang/lib/AST/Decl.cpp
@@ -1604,17 +1604,20 @@ LinkageInfo LinkageComputer::getLVForDecl(const NamedDecl *D,
   // We have just computed the linkage for this decl. By induction we know
   // that all other computed linkages match, check that the one we just
   // computed also does.
-  NamedDecl *Old = nullptr;
-  for (auto *I : D->redecls()) {
-    auto *T = cast<NamedDecl>(I);
-    if (T == D)
+  // We can't assume the redecl chain is well formed at this point,
+  // so keep track of already visited declarations.
+  for (llvm::SmallPtrSet<const Decl *, 4> AlreadyVisited{D}; /**/; /**/) {
+    D = cast<NamedDecl>(const_cast<NamedDecl *>(D)->getNextRedeclarationImpl());
+    if (!AlreadyVisited.insert(D).second)
+      break;
+    if (D->isInvalidDecl())
       continue;
-    if (!T->isInvalidDecl() && T->hasCachedLinkage()) {
-      Old = T;
+    if (auto OldLinkage = D->getCachedLinkage();
+        OldLinkage != Linkage::Invalid) {
+      assert(LV.getLinkage() == OldLinkage);
       break;
     }
   }
-  assert(!Old || Old->getCachedLinkage() == D->getCachedLinkage());
 #endif
 
   return LV;
diff --git a/clang/test/Modules/GH153933.cpp b/clang/test/Modules/GH153933.cpp
new file mode 100644
index 000000000000..41184c6b0060
--- /dev/null
+++ b/clang/test/Modules/GH153933.cpp
@@ -0,0 +1,23 @@
+// RUN: rm -rf %t
+// RUN: mkdir -p %t
+// RUN: split-file %s %t
+//
+// RUN: %clang_cc1 -std=c++20 %t/B.cppm -emit-module-interface -o %t/B.pcm
+// RUN: %clang_cc1 -std=c++20 -fsyntax-only -fprebuilt-module-path=%t %t/C.cpp
+
+//--- A.hpp
+template<class> struct A {};
+template<class T> struct B {
+  virtual A<T> v() { return {}; }
+};
+B<void> x;
+
+//--- B.cppm
+module;
+#include "A.hpp"
+export module B;
+using ::x;
+
+//--- C.cpp
+#include "A.hpp"
+import B;