Allows absl::StrFormat to accept types which implement AbslStringify()

PiperOrigin-RevId: 477507777
Change-Id: I5ecde3163ca560ac8774034e55654774e36ad230

1 files changed, 24 insertions, 10 deletions

diff --git a/absl/strings/internal/str_format/arg.h b/absl/strings/internal/str_format/arg.h
index 2dfbf728..3d616dba 100644
--- a/absl/strings/internal/str_format/arg.h
+++ b/absl/strings/internal/str_format/arg.h
@@ -45,6 +45,11 @@ class FormatConversionSpec;
 
 namespace str_format_internal {
 
+template <FormatConversionCharSet C>
+struct ArgConvertResult {
+  bool value;
+};
+
 template <typename T, typename = void>
 struct HasUserDefinedConvert : std::false_type {};
 
@@ -71,6 +76,20 @@ auto FormatConvertImpl(const T& v, FormatConversionSpecImpl conv,
   return AbslFormatConvert(v, fcs, &fs);
 }
 
+void AbslStringify();  // Stops the lexical name lookup
+template <typename T>
+auto FormatConvertImpl(const T& v, FormatConversionSpecImpl,
+                       FormatSinkImpl* sink)
+    -> std::enable_if_t<std::is_void<decltype(AbslStringify(
+                            std::declval<FormatSink&>(), v))>::value,
+                        ArgConvertResult<FormatConversionCharSetInternal::v>> {
+  using FormatSinkT =
+      absl::enable_if_t<sizeof(const T& (*)()) != 0, FormatSink>;
+  auto fs = sink->Wrap<FormatSinkT>();
+  AbslStringify(fs, v);
+  return {true};
+}
+
 template <typename T>
 class StreamedWrapper;
 
@@ -96,11 +115,6 @@ struct VoidPtr {
 };
 
 template <FormatConversionCharSet C>
-struct ArgConvertResult {
-  bool value;
-};
-
-template <FormatConversionCharSet C>
 constexpr FormatConversionCharSet ExtractCharSet(FormatConvertResult<C>) {
   return C;
 }
@@ -316,11 +330,11 @@ struct FormatArgImplFriend {
 
 template <typename Arg>
 constexpr FormatConversionCharSet ArgumentToConv() {
-  return absl::str_format_internal::ExtractCharSet(
-      decltype(str_format_internal::FormatConvertImpl(
-          std::declval<const Arg&>(),
-          std::declval<const FormatConversionSpecImpl&>(),
-          std::declval<FormatSinkImpl*>())){});
+  using ConvResult = decltype(str_format_internal::FormatConvertImpl(
+      std::declval<const Arg&>(),
+      std::declval<const FormatConversionSpecImpl&>(),
+      std::declval<FormatSinkImpl*>()));
+  return absl::str_format_internal::ExtractCharSet(ConvResult{});
 }
 
 // A type-erased handle to a format argument.