Index: include/clang/Sema/CodeCompleteConsumer.h
===================================================================
--- include/clang/Sema/CodeCompleteConsumer.h
+++ include/clang/Sema/CodeCompleteConsumer.h
@@ -18,6 +18,7 @@
 #include "clang/AST/DeclBase.h"
 #include "clang/AST/Type.h"
 #include "clang/Sema/CodeCompleteOptions.h"
+#include "clang/Sema/DeclSpec.h"
 #include "llvm/ADT/DenseMap.h"
 #include "llvm/ADT/SmallVector.h"
 #include "llvm/ADT/StringRef.h"
@@ -280,6 +281,10 @@
   /// \brief The identifiers for Objective-C selector parts.
   ArrayRef<IdentifierInfo *> SelIdents;
 
+  /// \brief The scope specifier that comes before the completion token e.g.
+  /// "a::b::"
+  llvm::Optional<CXXScopeSpec> ScopeSpecifier;
+
 public:
   /// \brief Construct a new code-completion context of the given kind.
   CodeCompletionContext(enum Kind Kind) : Kind(Kind), SelIdents(None) { }
@@ -315,8 +320,20 @@
   /// \brief Determines whether we want C++ constructors as results within this
   /// context.
   bool wantConstructorResults() const;
-};
 
+  /// \brief Sets the scope specifier that comes before the completion token.
+  /// This is expected to be set in code completions on qualfied specifiers
+  /// (e.g. "a::b::").
+  void setCXXScopeSpecifier(CXXScopeSpec SS) {
+    this->ScopeSpecifier = std::move(SS);
+  }
+
+  llvm::Optional<const CXXScopeSpec *> getCXXScopeSpecifier() {
+    if (ScopeSpecifier)
+      return ScopeSpecifier.getPointer();
+    return llvm::None;
+  }
+};
 
 /// \brief A "string" used to describe how code completion can
 /// be performed for an entity.
Index: lib/Sema/SemaCodeComplete.cpp
===================================================================
--- lib/Sema/SemaCodeComplete.cpp
+++ lib/Sema/SemaCodeComplete.cpp
@@ -4603,9 +4603,19 @@
 
 void Sema::CodeCompleteQualifiedId(Scope *S, CXXScopeSpec &SS,
                                    bool EnteringContext) {
-  if (!SS.getScopeRep() || !CodeCompleter)
+  if (SS.isEmpty() || !CodeCompleter)
     return;
 
+  // We want to keep the scope specifier even if it's invalid (e.g. the scope
+  // "a::b::" is not corresponding to any context/namespace in the AST), since
+  // it can be useful for global code completion which have information about
+  // contexts/symbols that are not in the AST.
+  if (SS.isInvalid()) {
+    CodeCompletionContext CC(CodeCompletionContext::CCC_Name);
+    CC.setCXXScopeSpecifier(SS);
+    HandleCodeCompleteResults(this, CodeCompleter, CC, nullptr, 0);
+    return;
+  }
   // Always pretend to enter a context to ensure that a dependent type
   // resolves to a dependent record.
   DeclContext *Ctx = computeDeclContext(SS, /*EnteringContext=*/true);
@@ -4621,7 +4631,7 @@
                         CodeCompleter->getCodeCompletionTUInfo(),
                         CodeCompletionContext::CCC_Name);
   Results.EnterNewScope();
-  
+
   // The "template" keyword can follow "::" in the grammar, but only
   // put it into the grammar if the nested-name-specifier is dependent.
   NestedNameSpecifier *NNS = SS.getScopeRep();
@@ -4635,16 +4645,18 @@
   // qualified-id completions.
   if (!EnteringContext)
     MaybeAddOverrideCalls(*this, Ctx, Results);
-  Results.ExitScope();  
-  
+  Results.ExitScope();
+
   CodeCompletionDeclConsumer Consumer(Results, CurContext);
   LookupVisibleDecls(Ctx, LookupOrdinaryName, Consumer,
                      /*IncludeGlobalScope=*/true,
                      /*IncludeDependentBases=*/true);
 
-  HandleCodeCompleteResults(this, CodeCompleter, 
-                            Results.getCompletionContext(),
-                            Results.data(),Results.size());
+  auto CC = Results.getCompletionContext();
+  CC.setCXXScopeSpecifier(SS);
+
+  HandleCodeCompleteResults(this, CodeCompleter, CC, Results.data(),
+                            Results.size());
 }
 
 void Sema::CodeCompleteUsing(Scope *S) {