From b2fa9f7b8e9dd0d82cb0ba2663617549e593d68c Mon Sep 17 00:00:00 2001
From: Jordan Rose <jordan_rose@apple.com>
Date: Mon, 7 Nov 2016 20:34:16 +0000
Subject: [PATCH 1/2] Disallow StringRef assignment from temporary
 std::strings.

Similar to r283798, this prevents accidentally referring to temporary
storage that goes out of scope by the end of the statement:

  someStringRef = getStringByValue();
  someStringRef = (Twine("-") + otherString).str();

Note that once again the constructor still has this problem:

  StringRef someStringRef = getStringByValue();

because once again we occasionally rely on this in calls:

  takesStringRef(getStringByValue());
  takesStringRef(Twine("-") + otherString);

Still, it's a step.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@286139 91177308-0d34-0410-b5e6-96231b3b80d8
---
 include/llvm/ADT/StringRef.h | 9 +++++++++
 1 file changed, 9 insertions(+)

diff --git a/include/llvm/ADT/StringRef.h b/include/llvm/ADT/StringRef.h
index a053b414a10..2c1883e5671 100644
--- a/include/llvm/ADT/StringRef.h
+++ b/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
     /// @{

From 25442627822d58f69d01945d65337dbb776f41d2 Mon Sep 17 00:00:00 2001
From: Jordan Rose <jordan_rose@apple.com>
Date: Mon, 7 Nov 2016 20:40:16 +0000
Subject: [PATCH 2/2] Add tests for r286139.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@286141 91177308-0d34-0410-b5e6-96231b3b80d8
---
 unittests/ADT/StringRefTest.cpp | 36 +++++++++++++++++++++++++++++++++
 1 file changed, 36 insertions(+)

diff --git a/unittests/ADT/StringRefTest.cpp b/unittests/ADT/StringRefTest.cpp
index ca6b011d695..9c65a4b3589 100644
--- a/unittests/ADT/StringRefTest.cpp
+++ b/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";