2 files changed, 25 insertions, 0 deletions
diff --git a/absl/strings/str_cat.h b/absl/strings/str_cat.h
index cba8ceb0..559ee0aa 100644
--- a/absl/strings/str_cat.h
+++ b/absl/strings/str_cat.h
@@ -56,6 +56,7 @@
 #include <cstdint>
 #include <string>
 #include <type_traits>
+#include <vector>
 
 #include "absl/base/port.h"
 #include "absl/strings/numbers.h"
@@ -268,6 +269,17 @@ class AlphaNum {
   AlphaNum(T e)  // NOLINT(runtime/explicit)
       : AlphaNum(static_cast<typename std::underlying_type<T>::type>(e)) {}
 
+  // vector<bool>::reference and const_reference require special help to
+  // convert to `AlphaNum` because it requires two user defined conversions.
+  template <
+      typename T,
+      typename std::enable_if<
+          std::is_class<T>::value &&
+          (std::is_same<T, std::vector<bool>::reference>::value ||
+           std::is_same<T, std::vector<bool>::const_reference>::value)>::type* =
+          nullptr>
+  AlphaNum(T e) : AlphaNum(static_cast<bool>(e)) {}  // NOLINT(runtime/explicit)
+
  private:
   absl::string_view piece_;
   char digits_[numbers_internal::kFastToBufferSize];
diff --git a/absl/strings/str_cat_test.cc b/absl/strings/str_cat_test.cc
index beb15fa9..1f1051d4 100644
--- a/absl/strings/str_cat_test.cc
+++ b/absl/strings/str_cat_test.cc
@@ -18,6 +18,7 @@
 
 #include <cstdint>
 #include <string>
+#include <vector>
 
 #include "gtest/gtest.h"
 #include "absl/strings/substitute.h"
@@ -395,6 +396,18 @@ TEST(StrAppend, Basics) {
             "No limit thanks to C++11's variadic templates");
 }
 
+TEST(StrCat, VectorBoolReferenceTypes) {
+  std::vector<bool> v;
+  v.push_back(true);
+  v.push_back(false);
+  std::vector<bool> const& cv = v;
+  // Test that vector<bool>::reference and vector<bool>::const_reference
+  // are handled as if the were really bool types and not the proxy types
+  // they really are.
+  std::string result = absl::StrCat(v[0], v[1], cv[0], cv[1]); // NOLINT
+  EXPECT_EQ(result, "1010");
+}
+
 #ifdef GTEST_HAS_DEATH_TEST
 TEST(StrAppend, Death) {
   std::string s = "self";