More tests for LWG#2156

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@253257 91177308-0d34-0410-b5e6-96231b3b80d8

test/std/containers/unord/unord.multimap/reserve.pass.cpp

@@ -33,6 +33,21 @@ void test(const C& c)
     assert(c.find(4)->second == "four");
 }
 
+void reserve_invariant(size_t n) // LWG #2156
+{
+    for (size_t i = 0; i < n; ++i)
+    {
+        std::unordered_multimap<size_t, size_t> c;
+        c.reserve(n);
+        size_t buckets = c.bucket_count();
+        for (size_t j = 0; j < i; ++j)
+        {
+            c[i] = i;
+            assert(buckets == c.bucket_count());
+        }
+    }
+}
+
 int main()
 {
     {
@@ -90,4 +105,5 @@ int main()
         test(c);
     }
 #endif
+    reserve_invariant(20);
 }