diff --git a/libcxx/include/__config b/libcxx/include/__config
--- a/libcxx/include/__config
+++ b/libcxx/include/__config
@@ -635,6 +635,10 @@
 // "C" function, as those lack mangling.
 #  define _LIBCPP_HIDE_FROM_ABI_C _LIBCPP_HIDDEN _LIBCPP_EXCLUDE_FROM_EXPLICIT_INSTANTIATION
 
+// This macro provides a HIDE_FROM_ABI equivalent for types.
+#  define _LIBCPP_HIDE_FROM_ABI_TYPE                                                                                   \
+    _LIBCPP_HIDDEN __attribute__((__abi_tag__(_LIBCPP_TOSTRING(_LIBCPP_VERSIONED_IDENTIFIER))))
+
 #  ifdef _LIBCPP_BUILDING_LIBRARY
 #    if _LIBCPP_ABI_VERSION > 1
 #      define _LIBCPP_HIDE_FROM_ABI_AFTER_V1 _LIBCPP_HIDE_FROM_ABI
diff --git a/libcxx/include/__format/parser_std_format_spec.h b/libcxx/include/__format/parser_std_format_spec.h
--- a/libcxx/include/__format/parser_std_format_spec.h
+++ b/libcxx/include/__format/parser_std_format_spec.h
@@ -105,8 +105,7 @@
 ///
 /// They default to false so when a new field is added it needs to be opted in
 /// explicitly.
-// TODO FMT Use an ABI tag for this struct.
-struct __fields {
+struct _LIBCPP_HIDE_FROM_ABI_TYPE __fields {
   uint8_t __sign_ : 1 {false};
   uint8_t __alternate_form_ : 1 {false};
   uint8_t __zero_padding_ : 1 {false};