diff --git a/llvm/include/llvm/ADT/StringRef.h b/llvm/include/llvm/ADT/StringRef.h
--- a/llvm/include/llvm/ADT/StringRef.h
+++ b/llvm/include/llvm/ADT/StringRef.h
@@ -149,11 +149,11 @@
 
     /// empty - Check if the string is empty.
     LLVM_NODISCARD
-    bool empty() const { return Length == 0; }
+    constexpr bool empty() const { return Length == 0; }
 
     /// size - Get the string size.
     LLVM_NODISCARD
-    size_t size() const { return Length; }
+    constexpr size_t size() const { return Length; }
 
     /// front - Get the first character in the string.
     LLVM_NODISCARD
diff --git a/llvm/unittests/ADT/StringRefTest.cpp b/llvm/unittests/ADT/StringRefTest.cpp
--- a/llvm/unittests/ADT/StringRefTest.cpp
+++ b/llvm/unittests/ADT/StringRefTest.cpp
@@ -1092,10 +1092,14 @@
 TEST(StringRefTest, StringLiteral) {
   constexpr StringRef StringRefs[] = {"Foo", "Bar"};
   EXPECT_EQ(StringRef("Foo"), StringRefs[0]);
+  EXPECT_EQ(3u, (std::integral_constant<size_t, StringRefs[0].size()>::value));
+  EXPECT_EQ(false, (std::integral_constant<bool, StringRefs[0].empty()>::value));
   EXPECT_EQ(StringRef("Bar"), StringRefs[1]);
 
   constexpr StringLiteral Strings[] = {"Foo", "Bar"};
   EXPECT_EQ(StringRef("Foo"), Strings[0]);
+  EXPECT_EQ(3u, (std::integral_constant<size_t, Strings[0].size()>::value));
+  EXPECT_EQ(false, (std::integral_constant<bool, Strings[0].empty()>::value));
   EXPECT_EQ(StringRef("Bar"), Strings[1]);
 }