diff --git a/clang/docs/ReleaseNotes.rst b/clang/docs/ReleaseNotes.rst
index 3f97372116d2b..46b6b06d27b0e 100644
--- a/clang/docs/ReleaseNotes.rst
+++ b/clang/docs/ReleaseNotes.rst
@@ -477,6 +477,10 @@ Improvements to Clang's diagnostics
 
 Bug Fixes in This Version
 -------------------------
+- Fixed an issue where a class template specialization whose declaration is
+  instantiated in one module and whose definition is instantiated in another
+  module may end up with members associated with the wrong declaration of the
+  class, which can result in miscompiles in some cases.
 
 - Added a new diagnostic warning group
   ``-Wdeprecated-redundant-constexpr-static-def``, under the existing
diff --git a/clang/lib/Serialization/ASTReaderDecl.cpp b/clang/lib/Serialization/ASTReaderDecl.cpp
index 10c92f8d21494..c8cbee14be4f0 100644
--- a/clang/lib/Serialization/ASTReaderDecl.cpp
+++ b/clang/lib/Serialization/ASTReaderDecl.cpp
@@ -181,6 +181,13 @@ namespace clang {
     static void setAnonymousDeclForMerging(ASTReader &Reader, DeclContext *DC,
                                            unsigned Index, NamedDecl *D);
 
+    /// Commit to a primary definition of the class RD, which is known to be
+    /// a definition of the class. We might not have read the definition data
+    /// for it yet. If we haven't then allocate placeholder definition data
+    /// now too.
+    static CXXRecordDecl *getOrFakePrimaryClassDefinition(ASTReader &Reader,
+                                                          CXXRecordDecl *RD);
+
     /// Results from loading a RedeclarableDecl.
     class RedeclarableResult {
       Decl *MergeWith;
@@ -598,7 +605,13 @@ void ASTDeclReader::VisitDecl(Decl *D) {
     auto *LexicalDC = readDeclAs<DeclContext>();
     if (!LexicalDC)
       LexicalDC = SemaDC;
-    DeclContext *MergedSemaDC = Reader.MergedDeclContexts.lookup(SemaDC);
+    // If the context is a class, we might not have actually merged it yet, in
+    // the case where the definition comes from an update record.
+    DeclContext *MergedSemaDC;
+    if (auto *RD = dyn_cast<CXXRecordDecl>(SemaDC))
+      MergedSemaDC = getOrFakePrimaryClassDefinition(Reader, RD);
+    else
+      MergedSemaDC = Reader.MergedDeclContexts.lookup(SemaDC);
     // Avoid calling setLexicalDeclContext() directly because it uses
     // Decl::getASTContext() internally which is unsafe during derialization.
     D->setDeclContextsImpl(MergedSemaDC ? MergedSemaDC : SemaDC, LexicalDC,
@@ -3198,6 +3211,32 @@ uint64_t ASTReader::getGlobalBitOffset(ModuleFile &M, uint64_t LocalOffset) {
   return LocalOffset + M.GlobalBitOffset;
 }
 
+CXXRecordDecl *
+ASTDeclReader::getOrFakePrimaryClassDefinition(ASTReader &Reader,
+                                               CXXRecordDecl *RD) {
+  // Try to dig out the definition.
+  auto *DD = RD->DefinitionData;
+  if (!DD)
+    DD = RD->getCanonicalDecl()->DefinitionData;
+
+  // If there's no definition yet, then DC's definition is added by an update
+  // record, but we've not yet loaded that update record. In this case, we
+  // commit to DC being the canonical definition now, and will fix this when
+  // we load the update record.
+  if (!DD) {
+    DD = new (Reader.getContext()) struct CXXRecordDecl::DefinitionData(RD);
+    RD->setCompleteDefinition(true);
+    RD->DefinitionData = DD;
+    RD->getCanonicalDecl()->DefinitionData = DD;
+
+    // Track that we did this horrible thing so that we can fix it later.
+    Reader.PendingFakeDefinitionData.insert(
+        std::make_pair(DD, ASTReader::PendingFakeDefinitionKind::Fake));
+  }
+
+  return DD->Definition;
+}
+
 /// Find the context in which we should search for previous declarations when
 /// looking for declarations to merge.
 DeclContext *ASTDeclReader::getPrimaryContextForMerging(ASTReader &Reader,
@@ -3205,29 +3244,8 @@ DeclContext *ASTDeclReader::getPrimaryContextForMerging(ASTReader &Reader,
   if (auto *ND = dyn_cast<NamespaceDecl>(DC))
     return ND->getOriginalNamespace();
 
-  if (auto *RD = dyn_cast<CXXRecordDecl>(DC)) {
-    // Try to dig out the definition.
-    auto *DD = RD->DefinitionData;
-    if (!DD)
-      DD = RD->getCanonicalDecl()->DefinitionData;
-
-    // If there's no definition yet, then DC's definition is added by an update
-    // record, but we've not yet loaded that update record. In this case, we
-    // commit to DC being the canonical definition now, and will fix this when
-    // we load the update record.
-    if (!DD) {
-      DD = new (Reader.getContext()) struct CXXRecordDecl::DefinitionData(RD);
-      RD->setCompleteDefinition(true);
-      RD->DefinitionData = DD;
-      RD->getCanonicalDecl()->DefinitionData = DD;
-
-      // Track that we did this horrible thing so that we can fix it later.
-      Reader.PendingFakeDefinitionData.insert(
-          std::make_pair(DD, ASTReader::PendingFakeDefinitionKind::Fake));
-    }
-
-    return DD->Definition;
-  }
+  if (auto *RD = dyn_cast<CXXRecordDecl>(DC))
+    return getOrFakePrimaryClassDefinition(Reader, RD);
 
   if (auto *RD = dyn_cast<RecordDecl>(DC))
     return RD->getDefinition();