[clang] fix redecl chain assumption when checking linkage consistency (llvm#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
llvm#147835, which was never
released, so there are no release notes for this.

Fixes llvm#153933

Author: mizvekov

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;

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;