Support passing I/O manipulators such as std::endl to StringifyStream

PiperOrigin-RevId: 892740803
Change-Id: I7c0c2d330080492d1451960bc21c04e6dbcbeb22

Author: Abseil Team

absl/strings/internal/stringify_stream.h

@@ -82,6 +82,13 @@ class StringifyStream {
     return stream << t;
   }
 
+  // Overload for things like << std::endl which need an explicit type in order
+  // to resolve to the appropriate overload or template instantiation.
+  StringifyStream& operator<<(std::ostream& (*func)(std::ostream&)) {
+    sink_.os << func;
+    return *this;
+  }
+
  private:
   // Abseil "stringify sink" concept (stringify_sink.h)
   struct Sink {

absl/strings/internal/stringify_stream_test.cc

@@ -15,6 +15,7 @@
 #include "absl/strings/internal/stringify_stream.h"
 
 #include <cstddef>
+#include <iomanip>
 #include <ostream>
 #include <sstream>
 
@@ -93,6 +94,16 @@ TEST(StringifyStreamTest, PreferStreamInsertionOverAbslStringify) {
   StringifyStream(os) << PreferStreamInsertionOverAbslStringifyTest{};
   EXPECT_EQ(os.str(), "good");
 }
+TEST(StringifyStreamTest, SupportEndl) {
+  std::ostringstream os;
+  StringifyStream(os) << std::endl;
+  EXPECT_EQ(os.str(), "\n");
+}
+TEST(StringifyStreamTest, SupportSetbase) {
+  std::ostringstream os;
+  StringifyStream(os) << std::setbase(16) << 255;
+  EXPECT_EQ(os.str(), "ff");
+}
 
 }  // namespace
 }  // namespace strings_internal