[Serialization] Stop demote var definition as declaration

clang/lib/Serialization/ASTReaderDecl.cpp

@@ -3641,23 +3641,9 @@ template<>
 void ASTDeclReader::attachPreviousDeclImpl(ASTReader &Reader,
                                            Redeclarable<VarDecl> *D,
                                            Decl *Previous, Decl *Canon) {
-  auto *VD = static_cast<VarDecl *>(D);
   auto *PrevVD = cast<VarDecl>(Previous);
   D->RedeclLink.setPrevious(PrevVD);
   D->First = PrevVD->First;
-
-  // We should keep at most one definition on the chain.
-  // FIXME: Cache the definition once we've found it. Building a chain with
-  // N definitions currently takes O(N^2) time here.
-  if (VD->isThisDeclarationADefinition() == VarDecl::Definition) {
-    for (VarDecl *CurD = PrevVD; CurD; CurD = CurD->getPreviousDecl()) {
-      if (CurD->isThisDeclarationADefinition() == VarDecl::Definition) {
-        Reader.mergeDefinitionVisibility(CurD, VD);
-        VD->demoteThisDefinitionToDeclaration();
-        break;
-      }
-    }
-  }
 }
 
 static bool isUndeducedReturnType(QualType T) {

clang/test/Modules/pr172241.cppm

@@ -0,0 +1,47 @@
+// RUN: rm -rf %t
+// RUN: mkdir -p %t
+// RUN: split-file %s %t
+//
+// RUN: %clang_cc1 -std=c++20 -triple %itanium_abi_triple %t/m.cppm -emit-module-interface -o %t/m.pcm
+// RUN: %clang_cc1 -std=c++20 -triple %itanium_abi_triple %t/use.cpp -fmodule-file=m=%t/m.pcm -emit-llvm -o - | FileCheck %t/use.cpp
+//
+// RUN: %clang_cc1 -std=c++20 -triple %itanium_abi_triple %t/m.cppm -emit-reduced-module-interface -o %t/m.pcm
+// RUN: %clang_cc1 -std=c++20 -triple %itanium_abi_triple %t/use.cpp -fmodule-file=m=%t/m.pcm -emit-llvm -o - | FileCheck %t/use.cpp
+
+//--- header.h
+#pragma once
+
+template <unsigned T>
+class Templ {
+public:
+    void lock() { __set_locked_bit(); }
+
+private:
+    static constexpr auto __set_locked_bit = [](){};
+};
+
+class JT {
+public:
+    ~JT() {
+        Templ<4> state;
+        state.lock();
+    }
+};
+
+//--- m.cppm
+module;
+#include "header.h"
+export module m;
+export struct M {
+    JT jt;
+};
+//--- use.cpp
+#include "header.h"
+import m;
+
+int main() {
+    M m;
+    return 0;
+}
+
+// CHECK: @_ZN5TemplILj4EE16__set_locked_bitE = {{.*}}linkonce_odr