Document and test ability to use absl::Overload with generic lambdas.

PiperOrigin-RevId: 613141617
Change-Id: Ife68495d4a5951b734db7407fb99686cf45a5133

2 files changed, 51 insertions, 2 deletions

diff --git a/absl/functional/overload.h b/absl/functional/overload.h
index 4651f14b..732795fc 100644
--- a/absl/functional/overload.h
+++ b/absl/functional/overload.h
@@ -35,7 +35,14 @@
 //                        [](double) -> absl::string_view { return "double"; }),
 //                     v) == "int");
 //
-// Note: This requires C++17.
+// One of the lambda may specify overload for several types via generic lambda.
+//
+// absl::variant<std::string, int32_t, int64_t> v(int32_t{1});
+// assert(std::visit(absl::Overload(
+//     [](const std::string& s) { return s.size(); },
+//     [](const auto& s) { return sizeof(s); }), v) == 4);
+//
+// Note: absl::Overload requires C++17.
 
 #ifndef ABSL_FUNCTIONAL_OVERLOAD_H_
 #define ABSL_FUNCTIONAL_OVERLOAD_H_
diff --git a/absl/functional/overload_test.cc b/absl/functional/overload_test.cc
index 739c4c4c..040bf83f 100644
--- a/absl/functional/overload_test.cc
+++ b/absl/functional/overload_test.cc
@@ -30,7 +30,7 @@
 
 namespace {
 
-TEST(OverloadTest, DispatchConsidersType) {
+TEST(OverloadTest, DispatchConsidersTypeWithAutoFallback) {
   auto overloaded = absl::Overload(
       [](int v) -> std::string { return absl::StrCat("int ", v); },        //
       [](double v) -> std::string { return absl::StrCat("double ", v); },  //
@@ -103,6 +103,35 @@ TEST(OverloadTest, AmbiguousConversionNotInvocable) {
   static_assert(!std::is_invocable_v<decltype(overloaded), int>);
 }
 
+TEST(OverloadTest, AmbiguousConversionWithAutoNotInvocable) {
+  auto overloaded = absl::Overload(  //
+      [](auto a) { return a; },  //
+      [](auto c) { return c; }   //
+  );
+  static_assert(!std::is_invocable_v<decltype(overloaded), int>);
+}
+
+#if ABSL_INTERNAL_CPLUSPLUS_LANG >= 202002L
+
+TEST(OverloadTest, AmbiguousConversionWithAutoAndTemplateNotInvocable) {
+  auto overloaded = absl::Overload(  //
+      [](auto a) { return a; },  //
+      []<class T>(T c) { return c; }   //
+  );
+  static_assert(!std::is_invocable_v<decltype(overloaded), int>);
+}
+
+TEST(OverloadTest, DispatchConsidersTypeWithTemplateFallback) {
+  auto overloaded = absl::Overload(       //
+      [](int a) { return a; },            //
+      []<class T>(T c) { return c * 2; }  //
+  );
+  EXPECT_EQ(7, overloaded(7));
+  EXPECT_EQ(14.0, overloaded(7.0));
+}
+
+#endif  // ABSL_INTERNAL_CPLUSPLUS_LANG >= 202002L
+
 TEST(OverloadTest, DispatchConsidersSfinae) {
   auto overloaded = absl::Overload(                    //
       [](auto a) -> decltype(a + 1) { return a + 1; }  //
@@ -125,6 +154,19 @@ TEST(OverloadTest, VariantVisitDispatchesCorrectly) {
   EXPECT_EQ("string", absl::visit(overloaded, v));
 }
 
+TEST(OverloadTest, VariantVisitWithAutoFallbackDispatchesCorrectly) {
+  absl::variant<std::string, int32_t, int64_t> v(int32_t{1});
+  auto overloaded = absl::Overload(
+      [](const std::string& s) { return s.size(); },  //
+      [](const auto& s) { return sizeof(s); }         //
+  );
+  EXPECT_EQ(4, absl::visit(overloaded, v));
+  v = int64_t{1};
+  EXPECT_EQ(8, absl::visit(overloaded, v));
+  v = std::string("hello");
+  EXPECT_EQ(5, absl::visit(overloaded, v));
+}
+
 }  // namespace
 
 #endif