Fix ArenaVector::swap (#7115)

This was never right for over a decade, and just never used I suppose... it should
have been called "take" since it grabbed data from the other item and then set
that other item to empty. Fix it so it swaps properly.

2 files changed, 40 insertions, 0 deletions

diff --git a/test/gtest/CMakeLists.txt b/test/gtest/CMakeLists.txt
index 9b648fb65..c3d281f1c 100644
--- a/test/gtest/CMakeLists.txt
+++ b/test/gtest/CMakeLists.txt
@@ -2,6 +2,7 @@ include_directories(../../third_party/googletest/googletest/include)
 include_directories(../../src/wasm)
 
 set(unittest_SOURCES
+  arena.cpp
   binary-reader.cpp
   cfg.cpp
   dfa_minimization.cpp
diff --git a/test/gtest/arena.cpp b/test/gtest/arena.cpp
new file mode 100644
index 000000000..7453ea9f6
--- /dev/null
+++ b/test/gtest/arena.cpp
@@ -0,0 +1,39 @@
+#include "mixed_arena.h"
+#include "gtest/gtest.h"
+
+using ArenaTest = ::testing::Test;
+
+TEST_F(ArenaTest, Swap) {
+  MixedArena arena;
+
+  ArenaVector<int> a(arena);
+  a.push_back(10);
+  a.push_back(20);
+
+  ArenaVector<int> b(arena);
+
+  EXPECT_EQ(a.size(), 2U);
+  EXPECT_EQ(b.size(), 0U);
+
+  a.swap(b);
+
+  EXPECT_EQ(a.size(), 0U);
+  EXPECT_EQ(b.size(), 2U);
+
+  a.swap(b);
+
+  EXPECT_EQ(a.size(), 2U);
+  EXPECT_EQ(b.size(), 0U);
+
+  // Now reverse a and b. The swap should be the same.
+
+  b.swap(a);
+
+  EXPECT_EQ(a.size(), 0U);
+  EXPECT_EQ(b.size(), 2U);
+
+  b.swap(a);
+
+  EXPECT_EQ(a.size(), 2U);
+  EXPECT_EQ(b.size(), 0U);
+}