Support self-copy in small_vector

Summary:
[Folly] Support self-copy in `small_vector`.

And refactor move-assignment to look like the fixed copy-assignment for symmetry.

Reviewed By: Ahmed-Salama

Differential Revision: D8321122

fbshipit-source-id: 2c8b26ad6c4e538266082df29276b827cf4505fc

folly/small_vector.h

@@ -46,6 +46,7 @@
 
 #include <folly/ConstexprMath.h>
 #include <folly/FormatTraits.h>
+#include <folly/Likely.h>
 #include <folly/Portability.h>
 #include <folly/Traits.h>
 #include <folly/lang/Assume.h>
@@ -489,18 +490,19 @@ class small_vector : public detail::small_vector_base<
   }
 
   small_vector& operator=(small_vector const& o) {
-    assign(o.begin(), o.end());
+    if (FOLLY_LIKELY(this != &o)) {
+      assign(o.begin(), o.end());
+    }
     return *this;
   }
 
   small_vector& operator=(small_vector&& o) {
     // TODO: optimization:
     // if both are internal, use move assignment where possible
-    if (this == &o) {
-      return *this;
+    if (FOLLY_LIKELY(this != &o)) {
+      clear();
+      swap(o);
     }
-    clear();
-    swap(o);
     return *this;
   }
 

folly/test/small_vector_test.cpp

@@ -1099,3 +1099,23 @@ TEST(small_vector, NoHeapStorageForSortedVectorSet) {
   EXPECT_THROW(test.insert(30), std::length_error);
   EXPECT_EQ(test.size(), 2);
 }
+
+TEST(small_vector, SelfMoveAssignmentForVectorOfPair) {
+  folly::small_vector<std::pair<int, int>, 2> test;
+  test.emplace_back(13, 2);
+  EXPECT_EQ(test.size(), 1);
+  EXPECT_EQ(test[0].first, 13);
+  test = static_cast<decltype(test)&&>(test); // trick clang -Wself-move
+  EXPECT_EQ(test.size(), 1);
+  EXPECT_EQ(test[0].first, 13);
+}
+
+TEST(small_vector, SelfCopyAssignmentForVectorOfPair) {
+  folly::small_vector<std::pair<int, int>, 2> test;
+  test.emplace_back(13, 2);
+  EXPECT_EQ(test.size(), 1);
+  EXPECT_EQ(test[0].first, 13);
+  test = test;
+  EXPECT_EQ(test.size(), 1);
+  EXPECT_EQ(test[0].first, 13);
+}