overload: make the constructor constexpr

PiperOrigin-RevId: 637029703
Change-Id: I58b4c3a3f1aab3062b51d15c5b9fecfd3ca3193a

absl/functional/overload.h

@@ -62,7 +62,7 @@ struct Overload final : T... {
   // aggregate initialization. Before then we must provide a constructor that
   // aggregate initialization. Before then we must provide a constructor that
   // makes this work.
   // makes this work.
   //
   //
-  explicit Overload(T... ts) : T(std::move(ts))... {}
+  constexpr explicit Overload(T... ts) : T(std::move(ts))... {}
 };
 };
 
 
 // Before C++20, which added support for CTAD for aggregate types, we must also
 // Before C++20, which added support for CTAD for aggregate types, we must also

absl/functional/overload_test.cc

@@ -194,6 +194,20 @@ TEST(OverloadTest, UseWithParentheses) {
   EXPECT_EQ(5, absl::visit(overloaded, v));
   EXPECT_EQ(5, absl::visit(overloaded, v));
 }
 }
 
 
+TEST(OverloadTest, HasConstexprConstructor) {
+  constexpr auto overloaded = absl::Overload{
+      [](int v) { return absl::StrCat("int ", v); },
+      [](double v) { return absl::StrCat("double ", v); },
+      [](const char* v) { return absl::StrCat("const char* ", v); },
+      [](auto v) { return absl::StrCat("auto ", v); },
+  };
+
+  EXPECT_EQ("int 1", overloaded(1));
+  EXPECT_EQ("double 2.5", overloaded(2.5));
+  EXPECT_EQ("const char* hello", overloaded("hello"));
+  EXPECT_EQ("auto 1.5", overloaded(1.5f));
+}
+
 }  // namespace
 }  // namespace
 
 
 #endif
 #endif