From 69f7fe357a92e09c00fd0f535049c2c26f11ac6e Mon Sep 17 00:00:00 2001
From: Victor Zverovich <victor.zverovich@gmail.com>
Date: Fri, 23 Apr 2021 09:12:08 -0700
Subject: [PATCH 1/5] Prepare for adding compile-time checks in <format>

---
 stl/inc/format | 66 ++++++++++++++++++++++++++++++--------------------
 1 file changed, 40 insertions(+), 26 deletions(-)

diff --git a/stl/inc/format b/stl/inc/format
index e5a549e7749..1bfc69afe1e 100644
--- a/stl/inc/format
+++ b/stl/inc/format
@@ -2857,6 +2857,20 @@ _NODISCARD auto make_wformat_args(const _Args&... _Vals) {
     return _Format_arg_store<wformat_context, _Args...>{_Vals...};
 }
 
+template <class _CharT, class... _Args>
+struct _Fmt_basic_string {
+    basic_string_view<_CharT> _Str;
+
+    template <class _Ty, enable_if_t<convertible_to<const _Ty&, basic_string_view<_CharT>>, int> = 0>
+    constexpr _Fmt_basic_string(const _Ty& _Str_) : _Str{_Str_} {}
+};
+
+template <class... _Args>
+using _Fmt_string = _Fmt_basic_string<char, type_identity_t<_Args>...>;
+
+template <class... _Args>
+using _Fmt_wstring = _Fmt_basic_string<wchar_t, type_identity_t<_Args>...>;
+
 template <output_iterator<const char&> _OutputIt>
 _OutputIt vformat_to(_OutputIt _Out, const string_view _Fmt, const format_args _Args) {
     if constexpr (is_same_v<_OutputIt, _Fmt_it>) {
@@ -2962,23 +2976,23 @@ _NODISCARD inline wstring vformat(const locale& _Loc, const wstring_view _Fmt, c
 }
 
 template <class... _Types>
-_NODISCARD string format(const string_view _Fmt, const _Types&... _Args) {
-    return _STD vformat(_Fmt, _STD make_format_args(_Args...));
+_NODISCARD string format(const _Fmt_string<_Types...> _Fmt, const _Types&... _Args) {
+    return _STD vformat(_Fmt._Str, _STD make_format_args(_Args...));
 }
 
 template <class... _Types>
-_NODISCARD wstring format(const wstring_view _Fmt, const _Types&... _Args) {
-    return _STD vformat(_Fmt, _STD make_wformat_args(_Args...));
+_NODISCARD wstring format(const _Fmt_wstring<_Types...> _Fmt, const _Types&... _Args) {
+    return _STD vformat(_Fmt._Str, _STD make_wformat_args(_Args...));
 }
 
 template <class... _Types>
-_NODISCARD string format(const locale& _Loc, const string_view _Fmt, const _Types&... _Args) {
-    return _STD vformat(_Loc, _Fmt, _STD make_format_args(_Args...));
+_NODISCARD string format(const locale& _Loc, const _Fmt_string<_Types...> _Fmt, const _Types&... _Args) {
+    return _STD vformat(_Loc, _Fmt._Str, _STD make_format_args(_Args...));
 }
 
 template <class... _Types>
-_NODISCARD wstring format(const locale& _Loc, const wstring_view _Fmt, const _Types&... _Args) {
-    return _STD vformat(_Loc, _Fmt, _STD make_wformat_args(_Args...));
+_NODISCARD wstring format(const locale& _Loc, const _Fmt_wstring<_Types...> _Fmt, const _Types&... _Args) {
+    return _STD vformat(_Loc, _Fmt._Str, _STD make_wformat_args(_Args...));
 }
 
 template <class _OutputIt>
@@ -2988,62 +3002,62 @@ struct format_to_n_result {
 };
 
 template <output_iterator<const char&> _OutputIt, class... _Types>
-format_to_n_result<_OutputIt> format_to_n(
-    _OutputIt _Out, const iter_difference_t<_OutputIt> _Max, const string_view _Fmt, const _Types&... _Args) {
+format_to_n_result<_OutputIt> format_to_n(_OutputIt _Out, const iter_difference_t<_OutputIt> _Max,
+    const _Fmt_string<_Types...> _Fmt, const _Types&... _Args) {
     _Fmt_iterator_buffer<_OutputIt, char, _Fmt_fixed_buffer_traits> _Buf(_STD move(_Out), _Max);
-    _STD vformat_to(_Fmt_it{_Buf}, _Fmt, _STD make_format_args(_Args...));
+    _STD vformat_to(_Fmt_it{_Buf}, _Fmt._Str, _STD make_format_args(_Args...));
     return {.out = _Buf._Out(), .size = _Buf._Count()};
 }
 
 template <output_iterator<const wchar_t&> _OutputIt, class... _Types>
-format_to_n_result<_OutputIt> format_to_n(
-    _OutputIt _Out, const iter_difference_t<_OutputIt> _Max, const wstring_view _Fmt, const _Types&... _Args) {
+format_to_n_result<_OutputIt> format_to_n(_OutputIt _Out, const iter_difference_t<_OutputIt> _Max,
+    const _Fmt_wstring<_Types...> _Fmt, const _Types&... _Args) {
     _Fmt_iterator_buffer<_OutputIt, wchar_t, _Fmt_fixed_buffer_traits> _Buf(_STD move(_Out), _Max);
-    _STD vformat_to(_Fmt_wit{_Buf}, _Fmt, _STD make_wformat_args(_Args...));
+    _STD vformat_to(_Fmt_wit{_Buf}, _Fmt._Str, _STD make_wformat_args(_Args...));
     return {.out = _Buf._Out(), .size = _Buf._Count()};
 }
 
 template <output_iterator<const char&> _OutputIt, class... _Types>
 format_to_n_result<_OutputIt> format_to_n(_OutputIt _Out, const iter_difference_t<_OutputIt> _Max, const locale& _Loc,
-    const string_view _Fmt, const _Types&... _Args) {
+    const _Fmt_string<_Types...> _Fmt, const _Types&... _Args) {
     _Fmt_iterator_buffer<_OutputIt, char, _Fmt_fixed_buffer_traits> _Buf(_STD move(_Out), _Max);
-    _STD vformat_to(_Fmt_it{_Buf}, _Loc, _Fmt, _STD make_format_args(_Args...));
+    _STD vformat_to(_Fmt_it{_Buf}, _Loc, _Fmt._Str, _STD make_format_args(_Args...));
     return {.out = _Buf._Out(), .size = _Buf._Count()};
 }
 
 template <output_iterator<const wchar_t&> _OutputIt, class... _Types>
 format_to_n_result<_OutputIt> format_to_n(_OutputIt _Out, const iter_difference_t<_OutputIt> _Max, const locale& _Loc,
-    const wstring_view _Fmt, const _Types&... _Args) {
+    const _Fmt_wstring<_Types...> _Fmt, const _Types&... _Args) {
     _Fmt_iterator_buffer<_OutputIt, wchar_t, _Fmt_fixed_buffer_traits> _Buf(_STD move(_Out), _Max);
-    _STD vformat_to(_Fmt_wit{_Buf}, _Loc, _Fmt, _STD make_wformat_args(_Args...));
+    _STD vformat_to(_Fmt_wit{_Buf}, _Loc, _Fmt._Str, _STD make_wformat_args(_Args...));
     return {.out = _Buf._Out(), .size = _Buf._Count()};
 }
 
 template <class... _Types>
-_NODISCARD size_t formatted_size(const string_view _Fmt, const _Types&... _Args) {
+_NODISCARD size_t formatted_size(const _Fmt_string<_Types...> _Fmt, const _Types&... _Args) {
     _Fmt_counting_buffer<char> _Buf;
-    _STD vformat_to(_Fmt_it{_Buf}, _Fmt, _STD make_format_args(_Args...));
+    _STD vformat_to(_Fmt_it{_Buf}, _Fmt._Str, _STD make_format_args(_Args...));
     return _Buf._Count();
 }
 
 template <class... _Types>
-_NODISCARD size_t formatted_size(const wstring_view _Fmt, const _Types&... _Args) {
+_NODISCARD size_t formatted_size(const _Fmt_wstring<_Types...> _Fmt, const _Types&... _Args) {
     _Fmt_counting_buffer<wchar_t> _Buf;
-    _STD vformat_to(_Fmt_wit{_Buf}, _Fmt, _STD make_wformat_args(_Args...));
+    _STD vformat_to(_Fmt_wit{_Buf}, _Fmt._Str, _STD make_wformat_args(_Args...));
     return _Buf._Count();
 }
 
 template <class... _Types>
-_NODISCARD size_t formatted_size(const locale& _Loc, const string_view _Fmt, const _Types&... _Args) {
+_NODISCARD size_t formatted_size(const locale& _Loc, const _Fmt_string<_Types...> _Fmt, const _Types&... _Args) {
     _Fmt_counting_buffer<char> _Buf;
-    _STD vformat_to(_Fmt_it{_Buf}, _Loc, _Fmt, _STD make_format_args(_Args...));
+    _STD vformat_to(_Fmt_it{_Buf}, _Loc, _Fmt._Str, _STD make_format_args(_Args...));
     return _Buf._Count();
 }
 
 template <class... _Types>
-_NODISCARD size_t formatted_size(const locale& _Loc, const wstring_view _Fmt, const _Types&... _Args) {
+_NODISCARD size_t formatted_size(const locale& _Loc, const _Fmt_wstring<_Types...> _Fmt, const _Types&... _Args) {
     _Fmt_counting_buffer<wchar_t> _Buf;
-    _STD vformat_to(_Fmt_wit{_Buf}, _Loc, _Fmt, _STD make_wformat_args(_Args...));
+    _STD vformat_to(_Fmt_wit{_Buf}, _Loc, _Fmt._Str, _STD make_wformat_args(_Args...));
     return _Buf._Count();
 }
 

From 20d8e8f27aaa8cc79af8ffcf9d21fc4e1a3bfc0c Mon Sep 17 00:00:00 2001
From: Victor Zverovich <victor.zverovich@gmail.com>
Date: Mon, 26 Apr 2021 18:03:37 -0700
Subject: [PATCH 2/5] Add comment, _Fmt_basic_string -> _Basic_format_string

---
 stl/inc/format | 9 +++++----
 1 file changed, 5 insertions(+), 4 deletions(-)

diff --git a/stl/inc/format b/stl/inc/format
index 1bfc69afe1e..a69c482d09c 100644
--- a/stl/inc/format
+++ b/stl/inc/format
@@ -2858,18 +2858,19 @@ _NODISCARD auto make_wformat_args(const _Args&... _Vals) {
 }
 
 template <class _CharT, class... _Args>
-struct _Fmt_basic_string {
+struct _Basic_format_string {
     basic_string_view<_CharT> _Str;
 
+    // This constructor should be consteval once there is a compiler support.
     template <class _Ty, enable_if_t<convertible_to<const _Ty&, basic_string_view<_CharT>>, int> = 0>
-    constexpr _Fmt_basic_string(const _Ty& _Str_) : _Str{_Str_} {}
+    constexpr _Basic_format_string(const _Ty& _Str_) : _Str{_Str_} {}
 };
 
 template <class... _Args>
-using _Fmt_string = _Fmt_basic_string<char, type_identity_t<_Args>...>;
+using _Fmt_string = _Basic_format_string<char, type_identity_t<_Args>...>;
 
 template <class... _Args>
-using _Fmt_wstring = _Fmt_basic_string<wchar_t, type_identity_t<_Args>...>;
+using _Fmt_wstring = _Basic_format_string<wchar_t, type_identity_t<_Args>...>;
 
 template <output_iterator<const char&> _OutputIt>
 _OutputIt vformat_to(_OutputIt _Out, const string_view _Fmt, const format_args _Args) {

From 1fc3171f48f4a94d692df868cd2127b7e65bc02b Mon Sep 17 00:00:00 2001
From: Victor Zverovich <victor.zverovich@gmail.com>
Date: Tue, 27 Apr 2021 21:22:02 -0700
Subject: [PATCH 3/5] Update comment

---
 stl/inc/format | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/stl/inc/format b/stl/inc/format
index a69c482d09c..5593fac86a4 100644
--- a/stl/inc/format
+++ b/stl/inc/format
@@ -2861,7 +2861,7 @@ template <class _CharT, class... _Args>
 struct _Basic_format_string {
     basic_string_view<_CharT> _Str;
 
-    // This constructor should be consteval once there is a compiler support.
+    // TRANSITION, consteval compiler support
     template <class _Ty, enable_if_t<convertible_to<const _Ty&, basic_string_view<_CharT>>, int> = 0>
     constexpr _Basic_format_string(const _Ty& _Str_) : _Str{_Str_} {}
 };

From b8c5676a0acf6cb8ea9e2dd0ca450f70b5720e68 Mon Sep 17 00:00:00 2001
From: Victor Zverovich <victor.zverovich@gmail.com>
Date: Thu, 29 Apr 2021 06:26:03 -0700
Subject: [PATCH 4/5] Use _Fmt_*string in format_to overloads

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

diff --git a/stl/inc/format b/stl/inc/format
index 5593fac86a4..43e40fb6bea 100644
--- a/stl/inc/format
+++ b/stl/inc/format
@@ -2929,23 +2929,23 @@ _OutputIt vformat_to(_OutputIt _Out, const locale& _Loc, const wstring_view _Fmt
 }
 
 template <output_iterator<const char&> _OutputIt, class... _Types>
-_OutputIt format_to(_OutputIt _Out, const string_view _Fmt, const _Types&... _Args) {
-    return _STD vformat_to(_STD move(_Out), _Fmt, _STD make_format_args(_Args...));
+_OutputIt format_to(_OutputIt _Out, const _Fmt_string<_Types...> _Fmt, const _Types&... _Args) {
+    return _STD vformat_to(_STD move(_Out), _Fmt._Str, _STD make_format_args(_Args...));
 }
 
 template <output_iterator<const wchar_t&> _OutputIt, class... _Types>
-_OutputIt format_to(_OutputIt _Out, const wstring_view _Fmt, const _Types&... _Args) {
-    return _STD vformat_to(_STD move(_Out), _Fmt, _STD make_wformat_args(_Args...));
+_OutputIt format_to(_OutputIt _Out, const _Fmt_wstring<_Types...> _Fmt, const _Types&... _Args) {
+    return _STD vformat_to(_STD move(_Out), _Fmt._Str, _STD make_wformat_args(_Args...));
 }
 
 template <output_iterator<const char&> _OutputIt, class... _Types>
-_OutputIt format_to(_OutputIt _Out, const locale& _Loc, const string_view _Fmt, const _Types&... _Args) {
-    return _STD vformat_to(_STD move(_Out), _Loc, _Fmt, _STD make_format_args(_Args...));
+_OutputIt format_to(_OutputIt _Out, const locale& _Loc, const _Fmt_string<_Types...> _Fmt, const _Types&... _Args) {
+    return _STD vformat_to(_STD move(_Out), _Loc, _Fmt._Str, _STD make_format_args(_Args...));
 }
 
 template <output_iterator<const wchar_t&> _OutputIt, class... _Types>
-_OutputIt format_to(_OutputIt _Out, const locale& _Loc, const wstring_view _Fmt, const _Types&... _Args) {
-    return _STD vformat_to(_STD move(_Out), _Loc, _Fmt, _STD make_wformat_args(_Args...));
+_OutputIt format_to(_OutputIt _Out, const locale& _Loc, const _Fmt_wstring<_Types...> _Fmt, const _Types&... _Args) {
+    return _STD vformat_to(_STD move(_Out), _Loc, _Fmt._Str, _STD make_wformat_args(_Args...));
 }
 
 _NODISCARD inline string vformat(const string_view _Fmt, const format_args _Args) {

From 09cb3f036c9abd30c3773bcb91ee9337d8e5a2f7 Mon Sep 17 00:00:00 2001
From: Victor Zverovich <victor.zverovich@gmail.com>
Date: Thu, 29 Apr 2021 07:14:43 -0700
Subject: [PATCH 5/5] Replace SFINAE with a concept check

---
 stl/inc/format | 5 +++--
 1 file changed, 3 insertions(+), 2 deletions(-)

diff --git a/stl/inc/format b/stl/inc/format
index 43e40fb6bea..0683f56dd28 100644
--- a/stl/inc/format
+++ b/stl/inc/format
@@ -2862,8 +2862,9 @@ struct _Basic_format_string {
     basic_string_view<_CharT> _Str;
 
     // TRANSITION, consteval compiler support
-    template <class _Ty, enable_if_t<convertible_to<const _Ty&, basic_string_view<_CharT>>, int> = 0>
-    constexpr _Basic_format_string(const _Ty& _Str_) : _Str{_Str_} {}
+    template <class _Ty>
+    requires convertible_to<const _Ty&, basic_string_view<_CharT>> constexpr _Basic_format_string(const _Ty& _Str_)
+        : _Str{_Str_} {}
 };
 
 template <class... _Args>