From a8aa70fb74ac6ba5b80f0e07291da1f4edcda551 Mon Sep 17 00:00:00 2001
From: "A. Jiang" <de34@live.cn>
Date: Sat, 11 Feb 2023 09:46:20 +0800
Subject: [PATCH 1/3] Make instantiation of `_Parse_format_string` lazy

---
 stl/inc/format | 24 ++++++++++++++++--------
 1 file changed, 16 insertions(+), 8 deletions(-)

diff --git a/stl/inc/format b/stl/inc/format
index 1e7feb962c5..7c01601b395 100644
--- a/stl/inc/format
+++ b/stl/inc/format
@@ -3417,8 +3417,10 @@ _NODISCARD auto make_wformat_args(_Args&&... _Vals) {
     return _Format_arg_store<wformat_context, _Args...>{_Vals...};
 }
 
-_EXPORT_STD template <output_iterator<const char&> _OutputIt>
-_OutputIt vformat_to(_OutputIt _Out, const string_view _Fmt, const format_args _Args) {
+_EXPORT_STD template <output_iterator<const char&> _OutputIt,
+    class _Sv = string_view> // lazy instantiation, see GH-1926
+_OutputIt vformat_to(_OutputIt _Out, const type_identity_t<_Sv> _Fmt, const format_args _Args) {
+    static_assert(is_same_v<_Sv, string_view>, "misspecified template argument");
     if constexpr (is_same_v<_OutputIt, _Fmt_it>) {
         _Format_handler<char> _Handler(_Out, _Fmt, _Args);
         _Parse_format_string(_Fmt, _Handler);
@@ -3431,8 +3433,10 @@ _OutputIt vformat_to(_OutputIt _Out, const string_view _Fmt, const format_args _
     }
 }
 
-_EXPORT_STD template <output_iterator<const wchar_t&> _OutputIt>
-_OutputIt vformat_to(_OutputIt _Out, const wstring_view _Fmt, const wformat_args _Args) {
+_EXPORT_STD template <output_iterator<const wchar_t&> _OutputIt,
+    class _Sv = wstring_view> // lazy instantiation, see GH-1926
+_OutputIt vformat_to(_OutputIt _Out, const type_identity_t<_Sv> _Fmt, const wformat_args _Args) {
+    static_assert(is_same_v<_Sv, wstring_view>, "misspecified template argument");
     if constexpr (is_same_v<_OutputIt, _Fmt_wit>) {
         _Format_handler<wchar_t> _Handler(_Out, _Fmt, _Args);
         _Parse_format_string(_Fmt, _Handler);
@@ -3445,8 +3449,10 @@ _OutputIt vformat_to(_OutputIt _Out, const wstring_view _Fmt, const wformat_args
     }
 }
 
-_EXPORT_STD template <output_iterator<const char&> _OutputIt>
-_OutputIt vformat_to(_OutputIt _Out, const locale& _Loc, const string_view _Fmt, const format_args _Args) {
+_EXPORT_STD template <output_iterator<const char&> _OutputIt,
+    class _Sv = string_view> // lazy instantiation, see GH-1926
+_OutputIt vformat_to(_OutputIt _Out, const locale& _Loc, const type_identity_t<_Sv> _Fmt, const format_args _Args) {
+    static_assert(is_same_v<_Sv, string_view>, "misspecified template argument");
     if constexpr (is_same_v<_OutputIt, _Fmt_it>) {
         _Format_handler<char> _Handler(_Out, _Fmt, _Args, _Lazy_locale{_Loc});
         _Parse_format_string(_Fmt, _Handler);
@@ -3459,8 +3465,10 @@ _OutputIt vformat_to(_OutputIt _Out, const locale& _Loc, const string_view _Fmt,
     }
 }
 
-_EXPORT_STD template <output_iterator<const wchar_t&> _OutputIt>
-_OutputIt vformat_to(_OutputIt _Out, const locale& _Loc, const wstring_view _Fmt, const wformat_args _Args) {
+_EXPORT_STD template <output_iterator<const wchar_t&> _OutputIt,
+    class _Sv = wstring_view> // lazy instantiation, see GH-1926
+_OutputIt vformat_to(_OutputIt _Out, const locale& _Loc, const type_identity_t<_Sv> _Fmt, const wformat_args _Args) {
+    static_assert(is_same_v<_Sv, wstring_view>, "misspecified template argument");
     if constexpr (is_same_v<_OutputIt, _Fmt_wit>) {
         _Format_handler<wchar_t> _Handler(_Out, _Fmt, _Args, _Lazy_locale{_Loc});
         _Parse_format_string(_Fmt, _Handler);

From b068cd4fddc7d2ed92a5a1ea42820cf5faef04d7 Mon Sep 17 00:00:00 2001
From: Casey Carter <Casey@Carter.net>
Date: Sun, 12 Feb 2023 00:34:46 -0800
Subject: [PATCH 2/3] Casey: try to avoid messing with the interface of
 `vformat_to`

---
 stl/inc/format | 48 ++++++++++++++++++++++++------------------------
 1 file changed, 24 insertions(+), 24 deletions(-)

diff --git a/stl/inc/format b/stl/inc/format
index 7c01601b395..fad0c7d4a9e 100644
--- a/stl/inc/format
+++ b/stl/inc/format
@@ -3417,65 +3417,65 @@ _NODISCARD auto make_wformat_args(_Args&&... _Vals) {
     return _Format_arg_store<wformat_context, _Args...>{_Vals...};
 }
 
-_EXPORT_STD template <output_iterator<const char&> _OutputIt,
-    class _Sv = string_view> // lazy instantiation, see GH-1926
-_OutputIt vformat_to(_OutputIt _Out, const type_identity_t<_Sv> _Fmt, const format_args _Args) {
-    static_assert(is_same_v<_Sv, string_view>, "misspecified template argument");
+_EXPORT_STD template <output_iterator<const char&> _OutputIt>
+_OutputIt vformat_to(_OutputIt _Out, const string_view _Fmt, const format_args _Args) {
+    // Make `_Parse_format_string` type-dependent to defer instantiation:
+    using _Dependent_char = conditional_t<is_same_v<_OutputIt, _Fmt_it>, char, char>;
     if constexpr (is_same_v<_OutputIt, _Fmt_it>) {
-        _Format_handler<char> _Handler(_Out, _Fmt, _Args);
+        _Format_handler<_Dependent_char> _Handler(_Out, _Fmt, _Args);
         _Parse_format_string(_Fmt, _Handler);
         return _Out;
     } else {
         _Fmt_iterator_buffer<_OutputIt, char> _Buf(_STD move(_Out));
-        _Format_handler<char> _Handler(_Fmt_it{_Buf}, _Fmt, _Args);
+        _Format_handler<_Dependent_char> _Handler(_Fmt_it{_Buf}, _Fmt, _Args);
         _Parse_format_string(_Fmt, _Handler);
         return _Buf._Out();
     }
 }
 
-_EXPORT_STD template <output_iterator<const wchar_t&> _OutputIt,
-    class _Sv = wstring_view> // lazy instantiation, see GH-1926
-_OutputIt vformat_to(_OutputIt _Out, const type_identity_t<_Sv> _Fmt, const wformat_args _Args) {
-    static_assert(is_same_v<_Sv, wstring_view>, "misspecified template argument");
+_EXPORT_STD template <output_iterator<const wchar_t&> _OutputIt>
+_OutputIt vformat_to(_OutputIt _Out, const wstring_view _Fmt, const wformat_args _Args) {
+    // Make `_Parse_format_string` type-dependent to defer instantiation:
+    using _Dependent_char = conditional_t<is_same_v<_OutputIt, _Fmt_wit>, wchar_t, wchar_t>;
     if constexpr (is_same_v<_OutputIt, _Fmt_wit>) {
-        _Format_handler<wchar_t> _Handler(_Out, _Fmt, _Args);
+        _Format_handler<_Dependent_char> _Handler(_Out, _Fmt, _Args);
         _Parse_format_string(_Fmt, _Handler);
         return _Out;
     } else {
         _Fmt_iterator_buffer<_OutputIt, wchar_t> _Buf(_STD move(_Out));
-        _Format_handler<wchar_t> _Handler(_Fmt_wit{_Buf}, _Fmt, _Args);
+        _Format_handler<_Dependent_char> _Handler(_Fmt_wit{_Buf}, _Fmt, _Args);
         _Parse_format_string(_Fmt, _Handler);
         return _Buf._Out();
     }
 }
 
-_EXPORT_STD template <output_iterator<const char&> _OutputIt,
-    class _Sv = string_view> // lazy instantiation, see GH-1926
-_OutputIt vformat_to(_OutputIt _Out, const locale& _Loc, const type_identity_t<_Sv> _Fmt, const format_args _Args) {
-    static_assert(is_same_v<_Sv, string_view>, "misspecified template argument");
+_EXPORT_STD template <output_iterator<const char&> _OutputIt>
+_OutputIt vformat_to(_OutputIt _Out, const locale& _Loc, const string_view _Fmt, const format_args _Args) {
+    // Make `_Parse_format_string` type-dependent to defer instantiation:
+    using _Dependent_char = conditional_t<is_same_v<_OutputIt, _Fmt_it>, char, char>;
     if constexpr (is_same_v<_OutputIt, _Fmt_it>) {
-        _Format_handler<char> _Handler(_Out, _Fmt, _Args, _Lazy_locale{_Loc});
+        _Format_handler<_Dependent_char> _Handler(_Out, _Fmt, _Args, _Lazy_locale{_Loc});
         _Parse_format_string(_Fmt, _Handler);
         return _Out;
     } else {
         _Fmt_iterator_buffer<_OutputIt, char> _Buf(_STD move(_Out));
-        _Format_handler<char> _Handler(_Fmt_it{_Buf}, _Fmt, _Args, _Lazy_locale{_Loc});
+        _Format_handler<_Dependent_char> _Handler(_Fmt_it{_Buf}, _Fmt, _Args, _Lazy_locale{_Loc});
         _Parse_format_string(_Fmt, _Handler);
         return _Buf._Out();
     }
 }
 
-_EXPORT_STD template <output_iterator<const wchar_t&> _OutputIt,
-    class _Sv = wstring_view> // lazy instantiation, see GH-1926
-_OutputIt vformat_to(_OutputIt _Out, const locale& _Loc, const type_identity_t<_Sv> _Fmt, const wformat_args _Args) {
-    static_assert(is_same_v<_Sv, wstring_view>, "misspecified template argument");
+_EXPORT_STD template <output_iterator<const wchar_t&> _OutputIt>
+_OutputIt vformat_to(_OutputIt _Out, const locale& _Loc, const wstring_view _Fmt, const wformat_args _Args) {
+    // Make `_Parse_format_string` type-dependent to defer instantiation:
+    using _Dependent_char = conditional_t<is_same_v<_OutputIt, _Fmt_wit>, wchar_t, wchar_t>;
     if constexpr (is_same_v<_OutputIt, _Fmt_wit>) {
-        _Format_handler<wchar_t> _Handler(_Out, _Fmt, _Args, _Lazy_locale{_Loc});
+        _Format_handler<_Dependent_char> _Handler(_Out, _Fmt, _Args, _Lazy_locale{_Loc});
         _Parse_format_string(_Fmt, _Handler);
         return _Out;
     } else {
         _Fmt_iterator_buffer<_OutputIt, wchar_t> _Buf(_STD move(_Out));
-        _Format_handler<wchar_t> _Handler(_Fmt_wit{_Buf}, _Fmt, _Args, _Lazy_locale{_Loc});
+        _Format_handler<_Dependent_char> _Handler(_Fmt_wit{_Buf}, _Fmt, _Args, _Lazy_locale{_Loc});
         _Parse_format_string(_Fmt, _Handler);
         return _Buf._Out();
     }

From c8ca089c296b0e72043411757914a7afaff02196 Mon Sep 17 00:00:00 2001
From: "A. Jiang" <de34@live.cn>
Date: Mon, 13 Feb 2023 00:22:27 +0800
Subject: [PATCH 3/3] Simplify `_Dependent_char`

---
 stl/inc/format | 8 ++++----
 1 file changed, 4 insertions(+), 4 deletions(-)

diff --git a/stl/inc/format b/stl/inc/format
index fad0c7d4a9e..9e3a41df7d1 100644
--- a/stl/inc/format
+++ b/stl/inc/format
@@ -3420,7 +3420,7 @@ _NODISCARD auto make_wformat_args(_Args&&... _Vals) {
 _EXPORT_STD template <output_iterator<const char&> _OutputIt>
 _OutputIt vformat_to(_OutputIt _Out, const string_view _Fmt, const format_args _Args) {
     // Make `_Parse_format_string` type-dependent to defer instantiation:
-    using _Dependent_char = conditional_t<is_same_v<_OutputIt, _Fmt_it>, char, char>;
+    using _Dependent_char = decltype((void) _Out, char{});
     if constexpr (is_same_v<_OutputIt, _Fmt_it>) {
         _Format_handler<_Dependent_char> _Handler(_Out, _Fmt, _Args);
         _Parse_format_string(_Fmt, _Handler);
@@ -3436,7 +3436,7 @@ _OutputIt vformat_to(_OutputIt _Out, const string_view _Fmt, const format_args _
 _EXPORT_STD template <output_iterator<const wchar_t&> _OutputIt>
 _OutputIt vformat_to(_OutputIt _Out, const wstring_view _Fmt, const wformat_args _Args) {
     // Make `_Parse_format_string` type-dependent to defer instantiation:
-    using _Dependent_char = conditional_t<is_same_v<_OutputIt, _Fmt_wit>, wchar_t, wchar_t>;
+    using _Dependent_char = decltype((void) _Out, wchar_t{});
     if constexpr (is_same_v<_OutputIt, _Fmt_wit>) {
         _Format_handler<_Dependent_char> _Handler(_Out, _Fmt, _Args);
         _Parse_format_string(_Fmt, _Handler);
@@ -3452,7 +3452,7 @@ _OutputIt vformat_to(_OutputIt _Out, const wstring_view _Fmt, const wformat_args
 _EXPORT_STD template <output_iterator<const char&> _OutputIt>
 _OutputIt vformat_to(_OutputIt _Out, const locale& _Loc, const string_view _Fmt, const format_args _Args) {
     // Make `_Parse_format_string` type-dependent to defer instantiation:
-    using _Dependent_char = conditional_t<is_same_v<_OutputIt, _Fmt_it>, char, char>;
+    using _Dependent_char = decltype((void) _Out, char{});
     if constexpr (is_same_v<_OutputIt, _Fmt_it>) {
         _Format_handler<_Dependent_char> _Handler(_Out, _Fmt, _Args, _Lazy_locale{_Loc});
         _Parse_format_string(_Fmt, _Handler);
@@ -3468,7 +3468,7 @@ _OutputIt vformat_to(_OutputIt _Out, const locale& _Loc, const string_view _Fmt,
 _EXPORT_STD template <output_iterator<const wchar_t&> _OutputIt>
 _OutputIt vformat_to(_OutputIt _Out, const locale& _Loc, const wstring_view _Fmt, const wformat_args _Args) {
     // Make `_Parse_format_string` type-dependent to defer instantiation:
-    using _Dependent_char = conditional_t<is_same_v<_OutputIt, _Fmt_wit>, wchar_t, wchar_t>;
+    using _Dependent_char = decltype((void) _Out, wchar_t{});
     if constexpr (is_same_v<_OutputIt, _Fmt_wit>) {
         _Format_handler<_Dependent_char> _Handler(_Out, _Fmt, _Args, _Lazy_locale{_Loc});
         _Parse_format_string(_Fmt, _Handler);