Fix sorted_vector_types bulk insertion

Summary:
The `bulk_insert()` method left duplicates from the input range when the
container was empty.

Also, `bulk_insert()` relies on `std::inplace_merge()` to be stable, but
`libc++`s version of `std::inplace_merge()` was buggy until [August 2017](https://github.com/llvm-mirror/libcxx/commit/25a78dcd773ce509469a2b892adbeac0c230cdb9).
Since it is fixed upstream, and it is a library bug, I've not mitigated the bug.

Reviewed By: yfeldblum

Differential Revision: D6943195

fbshipit-source-id: 136caa52c21a9e0dbc09b0a47ad57e99d2fde380

folly/sorted_vector_types.h

@@ -184,6 +184,7 @@ void bulk_insert(
   }
   if (middle != cont.begin() && !cmp(*(middle - 1), *middle)) {
     std::inplace_merge(cont.begin(), middle, cont.end(), cmp);
+  }
   cont.erase(
       std::unique(
           cont.begin(),
@@ -193,7 +194,6 @@ void bulk_insert(
             return !cmp(a, b) && !cmp(b, a);
           }),
       cont.end());
-  }
 }
 
 template <typename Container, typename Compare>

folly/test/sorted_vector_test.cpp

@@ -714,3 +714,12 @@ TEST(SortedVectorTypes, TestMapCreationFromVector) {
   });
   EXPECT_EQ(contents, expected_contents);
 }
+
+TEST(SortedVectorTypes, TestBulkInsertionWithDuplicatesIntoEmptySet) {
+  sorted_vector_set<int> set;
+  {
+    std::vector<int> const vec = {0, 1, 0, 1};
+    set.insert(vec.begin(), vec.end());
+  }
+  EXPECT_THAT(set, testing::ElementsAreArray({0, 1}));
+}