Disallow ArrayRef assignment from temporaries.

Without this, the following statements will create ArrayRefs that
refer to temporary storage that goes out of scope by the end of the
line:

  someArrayRef = getSingleElement();
  someArrayRef = {elem1, elem2};

Note that the constructor still has this problem:

  ArrayRef<Element> someArrayRef = getSingleElement();
  ArrayRef<Element> someArrayRef = {elem1, elem2};

but that's a little harder to get rid of because we want to be able to
use this in calls:

  takesArrayRef(getSingleElement());
  takesArrayRef({elem1, elem2});

Part of rdar://problem/16375365. Reviewed by Duncan Exon Smith.

llvm-svn: 283798

1 files changed, 23 insertions, 0 deletions

diff --git a/llvm/unittests/ADT/ArrayRefTest.cpp b/llvm/unittests/ADT/ArrayRefTest.cpp
index 43e5005e62d..f046da8a833 100644
--- a/llvm/unittests/ADT/ArrayRefTest.cpp
+++ b/llvm/unittests/ADT/ArrayRefTest.cpp
@@ -31,6 +31,21 @@ static_assert(
     !std::is_convertible<ArrayRef<volatile int *>, ArrayRef<int *>>::value,
     "Removing volatile");
 
+// Check that we can't accidentally assign a temporary location to an ArrayRef.
+// (Unfortunately we can't make use of the same thing with constructors.)
+static_assert(
+    !std::is_assignable<ArrayRef<int *>, int *>::value,
+    "Assigning from single prvalue element");
+static_assert(
+    !std::is_assignable<ArrayRef<int *>, int * &&>::value,
+    "Assigning from single xvalue element");
+static_assert(
+    std::is_assignable<ArrayRef<int *>, int * &>::value,
+    "Assigning from single lvalue element");
+static_assert(
+    !std::is_assignable<ArrayRef<int *>, std::initializer_list<int *>>::value,
+    "Assigning from an initializer list");
+
 namespace {
 
 TEST(ArrayRefTest, AllocatorCopy) {
@@ -161,6 +176,14 @@ TEST(ArrayRefTest, InitializerList) {
   ArgTest12({1, 2});
 }
 
+TEST(ArrayRefTest, EmptyInitializerList) {
+  ArrayRef<int> A = {};
+  EXPECT_TRUE(A.empty());
+
+  A = {};
+  EXPECT_TRUE(A.empty());
+}
+
 // Test that makeArrayRef works on ArrayRef (no-op)
 TEST(ArrayRefTest, makeArrayRef) {
   static const int A1[] = {1, 2, 3, 4, 5, 6, 7, 8};