diff --git a/clang/include/clang/CrossTU/CrossTranslationUnit.h b/clang/include/clang/CrossTU/CrossTranslationUnit.h
--- a/clang/include/clang/CrossTU/CrossTranslationUnit.h
+++ b/clang/include/clang/CrossTU/CrossTranslationUnit.h
@@ -17,6 +17,7 @@
 #include "clang/AST/ASTImporterSharedState.h"
 #include "clang/Basic/LLVM.h"
 #include "llvm/ADT/DenseMap.h"
+#include "llvm/ADT/Optional.h"
 #include "llvm/ADT/SmallPtrSet.h"
 #include "llvm/ADT/StringMap.h"
 #include "llvm/Support/Error.h"
@@ -159,7 +160,7 @@
                                                    ASTUnit *Unit);
 
   /// Get a name to identify a named decl.
-  static std::string getLookupName(const NamedDecl *ND);
+  static llvm::Optional<std::string> getLookupName(const NamedDecl *ND);
 
   /// Emit diagnostics for the user for potential configuration errors.
   void emitCrossTUDiagnostics(const IndexError &IE);
diff --git a/clang/lib/CrossTU/CrossTranslationUnit.cpp b/clang/lib/CrossTU/CrossTranslationUnit.cpp
--- a/clang/lib/CrossTU/CrossTranslationUnit.cpp
+++ b/clang/lib/CrossTU/CrossTranslationUnit.cpp
@@ -193,12 +193,13 @@
 
 CrossTranslationUnitContext::~CrossTranslationUnitContext() {}
 
-std::string CrossTranslationUnitContext::getLookupName(const NamedDecl *ND) {
+llvm::Optional<std::string>
+CrossTranslationUnitContext::getLookupName(const NamedDecl *ND) {
   SmallString<128> DeclUSR;
   bool Ret = index::generateUSRForDecl(ND, DeclUSR);
-  (void)Ret;
-  assert(!Ret && "Unable to generate USR");
-  return DeclUSR.str();
+  if (Ret)
+    return {};
+  return std::string(DeclUSR.str());
 }
 
 /// Recursively visits the decls of a DeclContext, and returns one with the
@@ -218,7 +219,8 @@
     const T *ResultDecl;
     if (!ND || !hasBodyOrInit(ND, ResultDecl))
       continue;
-    if (getLookupName(ResultDecl) != LookupName)
+    llvm::Optional<std::string> ResultLookupName = getLookupName(ResultDecl);
+    if (!ResultLookupName || *ResultLookupName != LookupName)
       continue;
     return ResultDecl;
   }
@@ -233,12 +235,12 @@
   assert(!hasBodyOrInit(D) &&
          "D has a body or init in current translation unit!");
   ++NumGetCTUCalled;
-  const std::string LookupName = getLookupName(D);
-  if (LookupName.empty())
+  const llvm::Optional<std::string> LookupName = getLookupName(D);
+  if (!LookupName)
     return llvm::make_error<IndexError>(
         index_error_code::failed_to_generate_usr);
   llvm::Expected<ASTUnit *> ASTUnitOrError =
-      loadExternalAST(LookupName, CrossTUDir, IndexName, DisplayCTUProgress);
+      loadExternalAST(*LookupName, CrossTUDir, IndexName, DisplayCTUProgress);
   if (!ASTUnitOrError)
     return ASTUnitOrError.takeError();
   ASTUnit *Unit = *ASTUnitOrError;
@@ -294,7 +296,7 @@
   }
 
   TranslationUnitDecl *TU = Unit->getASTContext().getTranslationUnitDecl();
-  if (const T *ResultDecl = findDefInDeclContext<T>(TU, LookupName))
+  if (const T *ResultDecl = findDefInDeclContext<T>(TU, *LookupName))
     return importDefinition(ResultDecl, Unit);
   return llvm::make_error<IndexError>(index_error_code::failed_import);
 }
diff --git a/clang/test/Analysis/Inputs/ctu-other.cpp b/clang/test/Analysis/Inputs/ctu-other.cpp
--- a/clang/test/Analysis/Inputs/ctu-other.cpp
+++ b/clang/test/Analysis/Inputs/ctu-other.cpp
@@ -131,3 +131,17 @@
   const unsigned int b;
 };
 U extU = {.a = 4};
+
+class TestAnonUnionUSR {
+public:
+  inline float f(int value) {
+    union {
+      float f;
+      int i;
+    };
+    i = value;
+    return f;
+  }
+  static const int Test;
+};
+const int TestAnonUnionUSR::Test = 5;
diff --git a/clang/test/Analysis/Inputs/ctu-other.cpp.externalDefMap.txt b/clang/test/Analysis/Inputs/ctu-other.cpp.externalDefMap.txt
--- a/clang/test/Analysis/Inputs/ctu-other.cpp.externalDefMap.txt
+++ b/clang/test/Analysis/Inputs/ctu-other.cpp.externalDefMap.txt
@@ -25,3 +25,4 @@
 c:@extSubSCN ctu-other.cpp.ast
 c:@extSCC ctu-other.cpp.ast
 c:@extU ctu-other.cpp.ast
+c:@S@TestAnonUnionUSR@Test ctu-other.cpp.ast
diff --git a/clang/test/Analysis/ctu-main.cpp b/clang/test/Analysis/ctu-main.cpp
--- a/clang/test/Analysis/ctu-main.cpp
+++ b/clang/test/Analysis/ctu-main.cpp
@@ -112,6 +112,19 @@
   clang_analyzer_eval(obj->fvcl(1) == 8);      // expected-warning{{FALSE}} expected-warning{{TRUE}}
 }
 
+class TestAnonUnionUSR {
+public:
+  inline float f(int value) {
+    union {
+      float f;
+      int i;
+    };
+    i = value;
+    return f;
+  }
+  static const int Test;
+};
+
 int main() {
   clang_analyzer_eval(f(3) == 2); // expected-warning{{TRUE}}
   clang_analyzer_eval(f(4) == 3); // expected-warning{{TRUE}}
@@ -144,4 +157,5 @@
   clang_analyzer_eval(extSubSCN.a == 1); // expected-warning{{TRUE}}
   // clang_analyzer_eval(extSCC.a == 7); // TODO
   clang_analyzer_eval(extU.a == 4); // expected-warning{{TRUE}}
+  clang_analyzer_eval(TestAnonUnionUSR::Test == 5); // expected-warning{{TRUE}}
 }
diff --git a/clang/test/Analysis/func-mapping-test.cpp b/clang/test/Analysis/func-mapping-test.cpp
--- a/clang/test/Analysis/func-mapping-test.cpp
+++ b/clang/test/Analysis/func-mapping-test.cpp
@@ -41,3 +41,10 @@
 };
 U u = {.a = 6};
 // CHECK-DAG: c:@u
+
+// No USR can be generated for this.
+// Check for no crash in this case.
+static union {
+  float uf;
+  const int ui;
+};
diff --git a/clang/tools/clang-extdef-mapping/ClangExtDefMapGen.cpp b/clang/tools/clang-extdef-mapping/ClangExtDefMapGen.cpp
--- a/clang/tools/clang-extdef-mapping/ClangExtDefMapGen.cpp
+++ b/clang/tools/clang-extdef-mapping/ClangExtDefMapGen.cpp
@@ -76,7 +76,12 @@
 
 void MapExtDefNamesConsumer::addIfInMain(const DeclaratorDecl *DD,
                                          SourceLocation defStart) {
-  std::string LookupName = CrossTranslationUnitContext::getLookupName(DD);
+  llvm::Optional<std::string> LookupName =
+      CrossTranslationUnitContext::getLookupName(DD);
+  if (!LookupName)
+    return;
+  assert(!LookupName->empty() && "Lookup name should be non-empty.");
+
   if (CurrentFileName.empty()) {
     CurrentFileName =
         SM.getFileEntryForID(SM.getMainFileID())->tryGetRealPathName();
@@ -89,7 +94,7 @@
   case VisibleNoLinkage:
   case UniqueExternalLinkage:
     if (SM.isInMainFile(defStart))
-      Index[LookupName] = CurrentFileName;
+      Index[*LookupName] = CurrentFileName;
     break;
   default:
     break;