Index: include/llvm/ADT/APInt.h
===================================================================
--- include/llvm/ADT/APInt.h
+++ include/llvm/ADT/APInt.h
@@ -693,24 +693,16 @@
 
   /// @brief Move assignment operator.
   APInt &operator=(APInt &&that) {
-    if (!isSingleWord()) {
-      // The MSVC STL shipped in 2013 requires that self move assignment be a
-      // no-op.  Otherwise algorithms like stable_sort will produce answers
-      // where half of the output is left in a moved-from state.
-      if (this == &that)
-        return *this;
+    assert(this != &that && "Self-move not supported");
+    if (!isSingleWord())
       delete[] pVal;
-    }
 
     // Use memcpy so that type based alias analysis sees both VAL and pVal
     // as modified.
     memcpy(&VAL, &that.VAL, sizeof(uint64_t));
 
-    // If 'this == &that', avoid zeroing our own bitwidth by storing to 'that'
-    // first.
-    unsigned ThatBitWidth = that.BitWidth;
+    BitWidth = that.BitWidth;
     that.BitWidth = 0;
-    BitWidth = ThatBitWidth;
 
     return *this;
   }
Index: unittests/ADT/APIntTest.cpp
===================================================================
--- unittests/ADT/APIntTest.cpp
+++ unittests/ADT/APIntTest.cpp
@@ -1606,36 +1606,6 @@
   }
 }
 
-#if defined(__clang__)
-// Disable the pragma warning from versions of Clang without -Wself-move
-#pragma clang diagnostic push
-#pragma clang diagnostic ignored "-Wunknown-pragmas"
-// Disable the warning that triggers on exactly what is being tested.
-#pragma clang diagnostic push
-#pragma clang diagnostic ignored "-Wself-move"
-#endif
-TEST(APIntTest, SelfMoveAssignment) {
-  APInt X(32, 0xdeadbeef);
-  X = std::move(X);
-  EXPECT_EQ(32u, X.getBitWidth());
-  EXPECT_EQ(0xdeadbeefULL, X.getLimitedValue());
-
-  uint64_t Bits[] = {0xdeadbeefdeadbeefULL, 0xdeadbeefdeadbeefULL};
-  APInt Y(128, Bits);
-  Y = std::move(Y);
-  EXPECT_EQ(128u, Y.getBitWidth());
-  EXPECT_EQ(~0ULL, Y.getLimitedValue());
-  const uint64_t *Raw = Y.getRawData();
-  EXPECT_EQ(2u, Y.getNumWords());
-  EXPECT_EQ(0xdeadbeefdeadbeefULL, Raw[0]);
-  EXPECT_EQ(0xdeadbeefdeadbeefULL, Raw[1]);
-}
-#if defined(__clang__)
-#pragma clang diagnostic pop
-#pragma clang diagnostic pop
-#endif
-}
-
 TEST(APIntTest, reverseBits) {
   EXPECT_EQ(1, APInt(1, 1).reverseBits());
   EXPECT_EQ(0, APInt(1, 0).reverseBits());
@@ -2025,3 +1995,5 @@
   APInt C = GreatestCommonDivisor(A, B);
   EXPECT_EQ(C, HugePrime);
 }
+
+} // end anonymous namespace