3 files changed, 31 insertions, 19 deletions

diff --git a/src/mongo/db/multi_key_path_tracker.cpp b/src/mongo/db/multi_key_path_tracker.cpp
index 3265cf1019c..d1c2c1ca293 100644
--- a/src/mongo/db/multi_key_path_tracker.cpp
+++ b/src/mongo/db/multi_key_path_tracker.cpp
@@ -29,18 +29,40 @@
 
 #include "mongo/platform/basic.h"
 
+#include <sstream>
+
 #include "mongo/db/multi_key_path_tracker.h"
 
 #include "mongo/util/assert_util.h"
+#include "mongo/util/str.h"
 
 namespace mongo {
 
 const OperationContext::Decoration<MultikeyPathTracker> MultikeyPathTracker::get =
     OperationContext::declareDecoration<MultikeyPathTracker>();
 
+// static
+std::string MultikeyPathTracker::dumpMultikeyPaths(const MultikeyPaths& multikeyPaths) {
+    std::stringstream ss;
+
+    ss << "[ ";
+    for (const auto& multikeyComponents : multikeyPaths) {
+        ss << "[ ";
+        for (const auto& multikeyComponent : multikeyComponents) {
+            ss << multikeyComponent << " ";
+        }
+        ss << "] ";
+    }
+    ss << "]";
+
+    return ss.str();
+}
+
 void MultikeyPathTracker::mergeMultikeyPaths(MultikeyPaths* toMergeInto,
                                              const MultikeyPaths& newPaths) {
-    invariant(toMergeInto->size() == newPaths.size());
+    invariant(toMergeInto->size() == newPaths.size(),
+              str::stream() << "toMergeInto: " << dumpMultikeyPaths(*toMergeInto) << "; newPaths: "
+                            << dumpMultikeyPaths(newPaths));
     for (auto idx = std::size_t(0); idx < toMergeInto->size(); ++idx) {
         toMergeInto->at(idx).insert(newPaths[idx].begin(), newPaths[idx].end());
     }
diff --git a/src/mongo/db/multi_key_path_tracker.h b/src/mongo/db/multi_key_path_tracker.h
index 8058f134adf..1fb1ee20fac 100644
--- a/src/mongo/db/multi_key_path_tracker.h
+++ b/src/mongo/db/multi_key_path_tracker.h
@@ -56,6 +56,11 @@ class MultikeyPathTracker {
 public:
     static const OperationContext::Decoration<MultikeyPathTracker> get;
 
+    /**
+     * Returns a string representation of MultikeyPaths for logging.
+     */
+    static std::string dumpMultikeyPaths(const MultikeyPaths& multikeyPaths);
+
     static void mergeMultikeyPaths(MultikeyPaths* toMergeInto, const MultikeyPaths& newPaths);
 
     // Decoration requires a default constructor.
diff --git a/src/mongo/db/multi_key_path_tracker_test.cpp b/src/mongo/db/multi_key_path_tracker_test.cpp
index 7c19eb69dc9..580b69519f3 100644
--- a/src/mongo/db/multi_key_path_tracker_test.cpp
+++ b/src/mongo/db/multi_key_path_tracker_test.cpp
@@ -42,28 +42,13 @@
 namespace mongo {
 namespace {
 
-std::string dumpMultikeyPaths(const MultikeyPaths& multikeyPaths) {
-    std::stringstream ss;
-
-    ss << "[ ";
-    for (const auto multikeyComponents : multikeyPaths) {
-        ss << "[ ";
-        for (const auto multikeyComponent : multikeyComponents) {
-            ss << multikeyComponent << " ";
-        }
-        ss << "] ";
-    }
-    ss << "]";
-
-    return ss.str();
-}
-
 void assertMultikeyPathsAreEqual(const MultikeyPaths& actual, const MultikeyPaths& expected) {
     bool match = (expected == actual);
     if (!match) {
-        FAIL(str::stream() << "Expected: " << dumpMultikeyPaths(expected) << ", "
+        FAIL(str::stream() << "Expected: " << MultikeyPathTracker::dumpMultikeyPaths(expected)
+                           << ", "
                            << "Actual: "
-                           << dumpMultikeyPaths(actual));
+                           << MultikeyPathTracker::dumpMultikeyPaths(actual));
     }
     ASSERT(match);
 }