inlined_vector: Use trivial relocation for SwapInlinedElements

absl/container/inlined_vector_test.cc

@@ -304,6 +304,33 @@ TEST(UniquePtr, MoveAssign) {
   }
 }
 
+// Swapping containers of unique pointers should work fine, with no
+// leaks, despite the fact that unique pointers are trivially relocatable but
+// not trivially destructible.
+TEST(UniquePtr, Swap) {
+  for (size_t size1 = 0; size1 < 5; ++size1) {
+    for (size_t size2 = 0; size2 < 5; ++size2) {
+      absl::InlinedVector<std::unique_ptr<size_t>, 2> a;
+      absl::InlinedVector<std::unique_ptr<size_t>, 2> b;
+      for (size_t i = 0; i < size1; ++i) {
+        a.push_back(std::make_unique<size_t>(i + 10));
+      }
+      for (size_t i = 0; i < size2; ++i) {
+        b.push_back(std::make_unique<size_t>(i + 20));
+      }
+      a.swap(b);
+      ASSERT_THAT(a, SizeIs(size2));
+      ASSERT_THAT(b, SizeIs(size1));
+      for (size_t i = 0; i < a.size(); ++i) {
+        ASSERT_THAT(a[i], Pointee(i + 20));
+      }
+      for (size_t i = 0; i < b.size(); ++i) {
+        ASSERT_THAT(b[i], Pointee(i + 10));
+      }
+    }
+  }
+}
+
 // At the end of this test loop, the elements between [erase_begin, erase_end)
 // should have reference counts == 0, and all others elements should have
 // reference counts == 1.

absl/container/internal/inlined_vector.h

@@ -322,14 +322,13 @@ class Storage {
 
   // The policy to be used specifically when swapping inlined elements.
   using SwapInlinedElementsPolicy = absl::conditional_t<
-      // Fast path: if the value type can be trivially move constructed/assigned
-      // and destroyed, and we know the allocator doesn't do anything fancy,
-      // then it's safe for us to simply swap the bytes in the inline storage.
-      // It's as if we had move-constructed a temporary vector, move-assigned
-      // one to the other, then move-assigned the first from the temporary.
-      absl::conjunction<absl::is_trivially_move_constructible<ValueType<A>>,
-                        absl::is_trivially_move_assignable<ValueType<A>>,
-                        absl::is_trivially_destructible<ValueType<A>>,
+      // Fast path: if the value type can be trivially relocated, and we
+      // know the allocator doesn't do anything fancy, then it's safe for us
+      // to simply swap the bytes in the inline storage. It's as if we had
+      // relocated the first vector's elements into temporary storage,
+      // relocated the second's elements into the (now-empty) first's,
+      // and then relocated from temporary storage into the second.
+      absl::conjunction<absl::is_trivially_relocatable<ValueType<A>>,
                         std::is_same<A, std::allocator<ValueType<A>>>>::value,
       MemcpyPolicy,
       absl::conditional_t<IsSwapOk<A>::value, ElementwiseSwapPolicy,