diff --git a/llvm/lib/Support/APInt.cpp b/llvm/lib/Support/APInt.cpp
--- a/llvm/lib/Support/APInt.cpp
+++ b/llvm/lib/Support/APInt.cpp
@@ -345,7 +345,7 @@
 /// In the slow case, we know the result is large.
 APInt APInt::concatSlowCase(const APInt &NewLSB) const {
   unsigned NewWidth = getBitWidth() + NewLSB.getBitWidth();
-  APInt Result = NewLSB.zext(NewWidth);
+  APInt Result = NewLSB.zextOrSelf(NewWidth);
   Result.insertBits(*this, NewLSB.getBitWidth());
   return Result;
 }
@@ -360,9 +360,13 @@
 
 void APInt::insertBits(const APInt &subBits, unsigned bitPosition) {
   unsigned subBitWidth = subBits.getBitWidth();
-  assert(0 < subBitWidth && (subBitWidth + bitPosition) <= BitWidth &&
+  assert(subBitWidth >= 0 && (subBitWidth + bitPosition) <= BitWidth &&
          "Illegal bit insertion");
 
+  // inserting no bits is a noop.
+  if (subBitWidth == 0)
+    return;
+
   // Insertion is a direct copy.
   if (subBitWidth == BitWidth) {
     *this = subBits;
diff --git a/llvm/unittests/ADT/APIntTest.cpp b/llvm/unittests/ADT/APIntTest.cpp
--- a/llvm/unittests/ADT/APIntTest.cpp
+++ b/llvm/unittests/ADT/APIntTest.cpp
@@ -1912,6 +1912,9 @@
   EXPECT_EQ(i260.extractBits(64, 128).getZExtValue(), 0xFFFFFFFFFFFF0000ull);
   EXPECT_EQ(i260.extractBits(64, 192).getZExtValue(), 0xFFFFFFFFFFFFFFFFull);
   EXPECT_EQ(i260.extractBits(4, 256).getZExtValue(), 0x000000000000000Full);
+
+  // Zero width insert is a noop.
+  i31.insertBits(APInt::getZeroWidth(), 1);
 }
 
 TEST(APIntTest, insertBitsUInt64) {
@@ -2587,7 +2590,7 @@
   EXPECT_EQ(0xFFFFFFFF, val.truncOrSelf(64));
 }
 
-TEST(APIntTest, concatMSB) {
+TEST(APIntTest, concat) {
   APInt Int1(4, 0x1ULL);
   APInt Int3(4, 0x3ULL);
 
@@ -2597,6 +2600,11 @@
 
   APInt I64(64, 0x3ULL);
   EXPECT_EQ(I64, I64.concat(I64).lshr(64).trunc(64));
+
+  APInt I65(65, 0x3ULL);
+  APInt I0 = APInt::getZeroWidth();
+  EXPECT_EQ(I65, I65.concat(I0));
+  EXPECT_EQ(I65, I0.concat(I65));
 }
 
 TEST(APIntTest, multiply) {