diff --git a/llvm/include/llvm/Support/TrigramIndex.h b/llvm/include/llvm/Support/TrigramIndex.h
index f772deca0301..0bfac498393f 100644
--- a/llvm/include/llvm/Support/TrigramIndex.h
+++ b/llvm/include/llvm/Support/TrigramIndex.h
@@ -27,12 +27,12 @@
 #define LLVM_SUPPORT_TRIGRAMINDEX_H
 
 #include "llvm/ADT/SmallVector.h"
-#include "llvm/ADT/StringRef.h"
 #include <string>
 #include <unordered_map>
 #include <vector>
 
 namespace llvm {
+class StringRef;
 
 class TrigramIndex {
  public:
diff --git a/llvm/lib/Support/TrigramIndex.cpp b/llvm/lib/Support/TrigramIndex.cpp
index 4370adc9c3e0..40a20ccc6583 100644
--- a/llvm/lib/Support/TrigramIndex.cpp
+++ b/llvm/lib/Support/TrigramIndex.cpp
@@ -15,6 +15,7 @@
 //===----------------------------------------------------------------------===//
 
 #include "llvm/Support/TrigramIndex.h"
+#include "llvm/ADT/StringRef.h"
 #include <set>
 
 using namespace llvm;