Index: clang-tools-extra/trunk/clangd/IncludeFixer.h
===================================================================
--- clang-tools-extra/trunk/clangd/IncludeFixer.h
+++ clang-tools-extra/trunk/clangd/IncludeFixer.h
@@ -58,9 +58,7 @@
   struct UnresolvedName {
     std::string Name;   // E.g. "X" in foo::X.
     SourceLocation Loc; // Start location of the unresolved name.
-    // Lazily get the possible scopes of the unresolved name. `Sema` must be
-    // alive when this is called.
-    std::function<std::vector<std::string>()> GetScopes;
+    std::vector<std::string> Scopes; // Namespace scopes we should search in.
   };
 
   /// Records the last unresolved name seen by Sema.
Index: clang-tools-extra/trunk/clangd/IncludeFixer.cpp
===================================================================
--- clang-tools-extra/trunk/clangd/IncludeFixer.cpp
+++ clang-tools-extra/trunk/clangd/IncludeFixer.cpp
@@ -16,6 +16,7 @@
 #include "index/Symbol.h"
 #include "clang/AST/Decl.h"
 #include "clang/AST/DeclBase.h"
+#include "clang/AST/DeclarationName.h"
 #include "clang/AST/NestedNameSpecifier.h"
 #include "clang/AST/Type.h"
 #include "clang/Basic/Diagnostic.h"
@@ -34,6 +35,7 @@
 #include "llvm/ADT/DenseMap.h"
 #include "llvm/ADT/None.h"
 #include "llvm/ADT/Optional.h"
+#include "llvm/ADT/StringExtras.h"
 #include "llvm/ADT/StringRef.h"
 #include "llvm/ADT/StringSet.h"
 #include "llvm/Support/Error.h"
@@ -301,6 +303,24 @@
   return Result;
 }
 
+/// Returns all namespace scopes that the unqualified lookup would visit.
+std::vector<std::string>
+collectAccessibleScopes(Sema &Sem, const DeclarationNameInfo &Typo, Scope *S,
+                        Sema::LookupNameKind LookupKind) {
+  std::vector<std::string> Scopes;
+  VisitedContextCollector Collector;
+  Sem.LookupVisibleDecls(S, LookupKind, Collector,
+                         /*IncludeGlobalScope=*/false,
+                         /*LoadExternal=*/false);
+
+  Scopes.push_back("");
+  for (const auto *Ctx : Collector.takeVisitedContexts()) {
+    if (isa<NamespaceDecl>(Ctx))
+      Scopes.push_back(printNamespaceScope(*Ctx));
+  }
+  return Scopes;
+}
+
 class IncludeFixer::UnresolvedNameRecorder : public ExternalSemaSource {
 public:
   UnresolvedNameRecorder(llvm::Optional<UnresolvedName> &LastUnresolvedName)
@@ -321,48 +341,30 @@
     if (!isInsideMainFile(Typo.getLoc(), SemaPtr->SourceMgr))
       return clang::TypoCorrection();
 
-    // This is not done lazily because `SS` can get out of scope and it's
-    // relatively cheap.
     auto Extracted = extractUnresolvedNameCheaply(
         SemaPtr->SourceMgr, Typo, SS, SemaPtr->LangOpts,
         static_cast<Sema::LookupNameKind>(LookupKind) ==
             Sema::LookupNameKind::LookupNestedNameSpecifierName);
     if (!Extracted)
       return TypoCorrection();
-    auto CheapUnresolved = std::move(*Extracted);
+
     UnresolvedName Unresolved;
-    Unresolved.Name = CheapUnresolved.Name;
+    Unresolved.Name = Extracted->Name;
     Unresolved.Loc = Typo.getBeginLoc();
-
-    if (!CheapUnresolved.ResolvedScope && !S) // Give up if no scope available.
+    if (!Extracted->ResolvedScope && !S) // Give up if no scope available.
       return TypoCorrection();
 
-    auto *Sem = SemaPtr; // Avoid capturing `this`.
-    Unresolved.GetScopes = [Sem, CheapUnresolved, S, LookupKind]() {
-      std::vector<std::string> Scopes;
-      if (CheapUnresolved.ResolvedScope) {
-        Scopes.push_back(*CheapUnresolved.ResolvedScope);
-      } else {
-        assert(S);
-        // No scope specifier is specified. Collect all accessible scopes in the
-        // context.
-        VisitedContextCollector Collector;
-        Sem->LookupVisibleDecls(
-            S, static_cast<Sema::LookupNameKind>(LookupKind), Collector,
-            /*IncludeGlobalScope=*/false,
-            /*LoadExternal=*/false);
-
-        Scopes.push_back("");
-        for (const auto *Ctx : Collector.takeVisitedContexts())
-          if (isa<NamespaceDecl>(Ctx))
-            Scopes.push_back(printNamespaceScope(*Ctx));
-      }
+    if (Extracted->ResolvedScope)
+      Unresolved.Scopes.push_back(*Extracted->ResolvedScope);
+    else // no qualifier or qualifier is unresolved.
+      Unresolved.Scopes = collectAccessibleScopes(
+          *SemaPtr, Typo, S, static_cast<Sema::LookupNameKind>(LookupKind));
+
+    if (Extracted->UnresolvedScope) {
+      for (std::string &Scope : Unresolved.Scopes)
+        Scope += *Extracted->UnresolvedScope;
+    }
 
-      if (CheapUnresolved.UnresolvedScope)
-        for (auto &Scope : Scopes)
-          Scope.append(*CheapUnresolved.UnresolvedScope);
-      return Scopes;
-    };
     LastUnresolvedName = std::move(Unresolved);
 
     // Never return a valid correction to try to recover. Our suggested fixes
@@ -384,14 +386,13 @@
 std::vector<Fix> IncludeFixer::fixUnresolvedName() const {
   assert(LastUnresolvedName.hasValue());
   auto &Unresolved = *LastUnresolvedName;
-  std::vector<std::string> Scopes = Unresolved.GetScopes();
   vlog("Trying to fix unresolved name \"{0}\" in scopes: [{1}]",
-       Unresolved.Name, llvm::join(Scopes.begin(), Scopes.end(), ", "));
+       Unresolved.Name, llvm::join(Unresolved.Scopes, ", "));
 
   FuzzyFindRequest Req;
   Req.AnyScope = false;
   Req.Query = Unresolved.Name;
-  Req.Scopes = Scopes;
+  Req.Scopes = Unresolved.Scopes;
   Req.RestrictForCodeCompletion = true;
   Req.Limit = 100;
 
Index: clang-tools-extra/trunk/clangd/unittests/DiagnosticsTests.cpp
===================================================================
--- clang-tools-extra/trunk/clangd/unittests/DiagnosticsTests.cpp
+++ clang-tools-extra/trunk/clangd/unittests/DiagnosticsTests.cpp
@@ -797,6 +797,27 @@
                               "Add include \"x.h\" for symbol a::X")))));
 }
 
+TEST(IncludeFixerTest, NoCrashOnTemplateInstantiations) {
+  Annotations Test(R"cpp(
+    template <typename T> struct Templ {
+      template <typename U>
+      typename U::type operator=(const U &);
+    };
+
+    struct A {
+      Templ<char> s;
+      A() { [[a]]; } // this caused crashes if we compute scopes lazily.
+    };
+  )cpp");
+
+  auto TU = TestTU::withCode(Test.code());
+  auto Index = buildIndexWithSymbol({});
+  TU.ExternalIndex = Index.get();
+
+  EXPECT_THAT(TU.build().getDiagnostics(),
+              ElementsAre(Diag(Test.range(), "use of undeclared identifier 'a'")));
+}
+
 TEST(DiagsInHeaders, DiagInsideHeader) {
   Annotations Main(R"cpp(
     #include [["a.h"]]