Disallow StringRef assignment from temporary std::strings

include/llvm/ADT/StringRef.h

@@ -226,6 +226,15 @@ namespace llvm {
       return Data[Index];
     }
 
+    /// Disallow accidental assignment from a temporary std::string.
+    ///
+    /// The declaration here is extra complicated so that `stringRef = {}`
+    /// and `stringRef = "abc"` continue to select the move assignment operator.
+    template <typename T>
+    typename std::enable_if<std::is_same<T, std::string>::value,
+                            StringRef>::type &
+    operator=(T &&Str) = delete;
+
     /// @}
     /// @name Type Conversions
     /// @{

unittests/ADT/StringRefTest.cpp

@@ -32,6 +32,34 @@ std::ostream &operator<<(std::ostream &OS,
 
 }
 
+// Check that we can't accidentally assign a temporary std::string to a
+// StringRef. (Unfortunately we can't make use of the same thing with
+// constructors.)
+//
+// Disable this check under MSVC; even MSVC 2015 isn't consistent between
+// std::is_assignable and actually writing such an assignment.
+#if !defined(_MSC_VER)
+static_assert(
+    !std::is_assignable<StringRef, std::string>::value,
+    "Assigning from prvalue std::string");
+static_assert(
+    !std::is_assignable<StringRef, std::string &&>::value,
+    "Assigning from xvalue std::string");
+static_assert(
+    std::is_assignable<StringRef, std::string &>::value,
+    "Assigning from lvalue std::string");
+static_assert(
+    std::is_assignable<StringRef, const char *>::value,
+    "Assigning from prvalue C string");
+static_assert(
+    std::is_assignable<StringRef, const char * &&>::value,
+    "Assigning from xvalue C string");
+static_assert(
+    std::is_assignable<StringRef, const char * &>::value,
+    "Assigning from lvalue C string");
+#endif
+
+
 namespace {
 TEST(StringRefTest, Construction) {
   EXPECT_EQ("", StringRef());
@@ -40,6 +68,14 @@ TEST(StringRefTest, Construction) {
   EXPECT_EQ("hello", StringRef(std::string("hello")));
 }
 
+TEST(StringRefTest, EmptyInitializerList) {
+  StringRef S = {};
+  EXPECT_TRUE(S.empty());
+
+  S = {};
+  EXPECT_TRUE(S.empty());
+}
+
 TEST(StringRefTest, Iteration) {
   StringRef S("hello");
   const char *p = "hello";