Add AbslStringify support for enum types in Substitute.

PiperOrigin-RevId: 492481345
Change-Id: Ie77656ed334b54930ee852d31e2794a1fc58ce2f

absl/strings/substitute.h

diff --git a/absl/strings/substitute.h b/absl/strings/substitute.h
index 5c3f6ef..d6a5a69 100644
--- a/absl/strings/substitute.h
+++ b/absl/strings/substitute.h
@@ -203,7 +203,8 @@
   // This overload matches only scoped enums.
   template <typename T,
             typename = typename std::enable_if<
-                std::is_enum<T>{} && !std::is_convertible<T, int>{}>::type>
+                std::is_enum<T>{} && !std::is_convertible<T, int>{} &&
+                !strings_internal::HasAbslStringify<T>::value>::type>
   Arg(T value)  // NOLINT(google-explicit-constructor)
       : Arg(static_cast<typename std::underlying_type<T>::type>(value)) {}
 

absl/strings/substitute_test.cc

diff --git a/absl/strings/substitute_test.cc b/absl/strings/substitute_test.cc
index 9f04545..9cb37c3 100644
--- a/absl/strings/substitute_test.cc
+++ b/absl/strings/substitute_test.cc
@@ -253,6 +253,18 @@
                                         ScopedEnumUInt16::kEnum1));
 }
 
+enum class EnumWithStringify { Many = 0, Choices = 1 };
+
+template <typename Sink>
+void AbslStringify(Sink& sink, EnumWithStringify e) {
+  sink.Append(e == EnumWithStringify::Many ? "Many" : "Choices");
+}
+
+TEST(SubstituteTest, AbslStringifyWithEnum) {
+  const auto e = EnumWithStringify::Choices;
+  EXPECT_EQ(absl::Substitute("$0", e), "Choices");
+}
+
 #ifdef GTEST_HAS_DEATH_TEST
 
 TEST(SubstituteDeathTest, SubstituteDeath) {