[clang] [clang] fix redecl chain assumption when checking liknkage consistency (PR #153996)

[Matheus Izvekov via cfe-commits]

https://github.com/mizvekov created https://github.com/llvm/llvm-project/pull/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 #152845, which was never released, so there are no release notes for this.

Fixes 153933

>From 0e6bb8af76c4337bc09a431b44dffef676d11586 Mon Sep 17 00:00:00 2001
From: Matheus Izvekov <mizvekov at gmail.com>
Date: Sun, 17 Aug 2025 00:17:46 -0300
Subject: [PATCH] [clang] fix redecl chain assumption when checking liknkage
 consistency

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 #152845, 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 | 22 ++++++++++++++++++++++
 2 files changed, 32 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 12fe5516883eb..4507f415ce606 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 0000000000000..d1452b1b2ab97
--- /dev/null
+++ b/clang/test/Modules/GH153933.cpp
@@ -0,0 +1,22 @@
+// 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;
+
+//--- C.cpp
+#include "A.hpp"
+import B;