From 46aeb0d281b69e17f61984d2cc2b99b6a7fccc9c Mon Sep 17 00:00:00 2001
From: Jakub Mazurkiewicz <mazkuba3@gmail.com>
Date: Wed, 16 Nov 2022 19:42:18 +0100
Subject: [PATCH 1/2] Implement LWG-3515

---
 stl/inc/stacktrace                            |  7 ++--
 tests/std/test.lst                            |  1 +
 .../env.lst                                   |  4 ++
 .../test.compile.pass.cpp                     | 39 +++++++++++++++++++
 4 files changed, 47 insertions(+), 4 deletions(-)
 create mode 100644 tests/std/tests/LWG3515_stacktrace_ostream_operator_should_be_less_templatized/env.lst
 create mode 100644 tests/std/tests/LWG3515_stacktrace_ostream_operator_should_be_less_templatized/test.compile.pass.cpp

diff --git a/stl/inc/stacktrace b/stl/inc/stacktrace
index 0c9e184c73..77393c6348 100644
--- a/stl/inc/stacktrace
+++ b/stl/inc/stacktrace
@@ -339,13 +339,12 @@ _NODISCARD string to_string(const basic_stacktrace<_Alloc>& _St) {
     return _Result;
 }
 
-_EXPORT_STD template <class _CharT, class _Traits>
-basic_ostream<_CharT, _Traits>& operator<<(basic_ostream<_CharT, _Traits>& _Os, const stacktrace_entry& _Fx) {
+_EXPORT_STD ostream& operator<<(ostream& _Os, const stacktrace_entry& _Fx) {
     return _Os << _STD to_string(_Fx);
 }
 
-_EXPORT_STD template <class _CharT, class _Traits, class _Alloc>
-basic_ostream<_CharT, _Traits>& operator<<(basic_ostream<_CharT, _Traits>& _Os, const basic_stacktrace<_Alloc>& _St) {
+_EXPORT_STD template <class _Alloc>
+ostream& operator<<(ostream& _Os, const basic_stacktrace<_Alloc>& _St) {
     return _Os << _STD to_string(_St);
 }
 
diff --git a/tests/std/test.lst b/tests/std/test.lst
index 9af561348b..98e7b58b31 100644
--- a/tests/std/test.lst
+++ b/tests/std/test.lst
@@ -224,6 +224,7 @@ tests\LWG3146_excessive_unwrapping_ref_cref
 tests\LWG3234_math_special_overloads
 tests\LWG3422_seed_seq_ctors
 tests\LWG3480_directory_iterator_range
+tests\LWG3515_stacktrace_ostream_operator_should_be_less_templatized
 tests\LWG3610_iota_view_size_and_integer_class
 tests\P0019R8_atomic_ref
 tests\P0024R2_parallel_algorithms_adjacent_difference
diff --git a/tests/std/tests/LWG3515_stacktrace_ostream_operator_should_be_less_templatized/env.lst b/tests/std/tests/LWG3515_stacktrace_ostream_operator_should_be_less_templatized/env.lst
new file mode 100644
index 0000000000..642f530ffa
--- /dev/null
+++ b/tests/std/tests/LWG3515_stacktrace_ostream_operator_should_be_less_templatized/env.lst
@@ -0,0 +1,4 @@
+# Copyright (c) Microsoft Corporation.
+# SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+
+RUNALL_INCLUDE ..\usual_latest_matrix.lst
diff --git a/tests/std/tests/LWG3515_stacktrace_ostream_operator_should_be_less_templatized/test.compile.pass.cpp b/tests/std/tests/LWG3515_stacktrace_ostream_operator_should_be_less_templatized/test.compile.pass.cpp
new file mode 100644
index 0000000000..b6bb0188f1
--- /dev/null
+++ b/tests/std/tests/LWG3515_stacktrace_ostream_operator_should_be_less_templatized/test.compile.pass.cpp
@@ -0,0 +1,39 @@
+// Copyright (c) Microsoft Corporation.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+
+#include <concepts>
+#include <memory>
+#include <memory_resource>
+#include <ostream>
+#include <sstream>
+#include <stacktrace>
+#include <string>
+
+using namespace std;
+
+template <class Ostream, class Alloc = allocator<stacktrace_entry>>
+concept CanPrintStacktrace = requires(Ostream& os, const stacktrace_entry& f, const basic_stacktrace<Alloc>& st) {
+                                 os << f;
+                                 os << st;
+                             };
+
+template <class CharT>
+struct FancyCharTraits : char_traits<CharT> {};
+
+static_assert(CanPrintStacktrace<ostream>);
+static_assert(CanPrintStacktrace<ostringstream>);
+static_assert(CanPrintStacktrace<ostream, pmr::polymorphic_allocator<stacktrace_entry>>);
+
+static_assert(!CanPrintStacktrace<wostream>);
+static_assert(!CanPrintStacktrace<wostringstream>);
+static_assert(!CanPrintStacktrace<wostream, pmr::polymorphic_allocator<stacktrace_entry>>);
+
+using FancyCharStream = basic_ostream<char, FancyCharTraits<char>>;
+static_assert(!CanPrintStacktrace<FancyCharStream>);
+static_assert(!CanPrintStacktrace<FancyCharStream, pmr::polymorphic_allocator<stacktrace_entry>>);
+
+using FancyWcharStream = basic_ostream<wchar_t, FancyCharTraits<wchar_t>>;
+static_assert(!CanPrintStacktrace<FancyWcharStream>);
+static_assert(!CanPrintStacktrace<FancyWcharStream, pmr::polymorphic_allocator<stacktrace_entry>>);
+
+int main() {} // COMPILE-ONLY

From 8a8a967121c427ecc0b102d812a5e4aea2a61e78 Mon Sep 17 00:00:00 2001
From: Jakub Mazurkiewicz <mazkuba3@gmail.com>
Date: Wed, 16 Nov 2022 20:46:11 +0100
Subject: [PATCH 2/2] Casey's comments

---
 stl/inc/stacktrace                            |  3 +-
 tests/std/test.lst                            |  1 -
 .../env.lst                                   |  4 --
 .../test.compile.pass.cpp                     | 39 -------------------
 tests/std/tests/P0881R7_stacktrace/test.cpp   | 31 +++++++++++++++
 5 files changed, 33 insertions(+), 45 deletions(-)
 delete mode 100644 tests/std/tests/LWG3515_stacktrace_ostream_operator_should_be_less_templatized/env.lst
 delete mode 100644 tests/std/tests/LWG3515_stacktrace_ostream_operator_should_be_less_templatized/test.compile.pass.cpp

diff --git a/stl/inc/stacktrace b/stl/inc/stacktrace
index 77393c6348..69e12dd240 100644
--- a/stl/inc/stacktrace
+++ b/stl/inc/stacktrace
@@ -339,7 +339,8 @@ _NODISCARD string to_string(const basic_stacktrace<_Alloc>& _St) {
     return _Result;
 }
 
-_EXPORT_STD ostream& operator<<(ostream& _Os, const stacktrace_entry& _Fx) {
+_EXPORT_STD template <int = 0>
+ostream& operator<<(ostream& _Os, const stacktrace_entry& _Fx) {
     return _Os << _STD to_string(_Fx);
 }
 
diff --git a/tests/std/test.lst b/tests/std/test.lst
index 98e7b58b31..9af561348b 100644
--- a/tests/std/test.lst
+++ b/tests/std/test.lst
@@ -224,7 +224,6 @@ tests\LWG3146_excessive_unwrapping_ref_cref
 tests\LWG3234_math_special_overloads
 tests\LWG3422_seed_seq_ctors
 tests\LWG3480_directory_iterator_range
-tests\LWG3515_stacktrace_ostream_operator_should_be_less_templatized
 tests\LWG3610_iota_view_size_and_integer_class
 tests\P0019R8_atomic_ref
 tests\P0024R2_parallel_algorithms_adjacent_difference
diff --git a/tests/std/tests/LWG3515_stacktrace_ostream_operator_should_be_less_templatized/env.lst b/tests/std/tests/LWG3515_stacktrace_ostream_operator_should_be_less_templatized/env.lst
deleted file mode 100644
index 642f530ffa..0000000000
--- a/tests/std/tests/LWG3515_stacktrace_ostream_operator_should_be_less_templatized/env.lst
+++ /dev/null
@@ -1,4 +0,0 @@
-# Copyright (c) Microsoft Corporation.
-# SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
-
-RUNALL_INCLUDE ..\usual_latest_matrix.lst
diff --git a/tests/std/tests/LWG3515_stacktrace_ostream_operator_should_be_less_templatized/test.compile.pass.cpp b/tests/std/tests/LWG3515_stacktrace_ostream_operator_should_be_less_templatized/test.compile.pass.cpp
deleted file mode 100644
index b6bb0188f1..0000000000
--- a/tests/std/tests/LWG3515_stacktrace_ostream_operator_should_be_less_templatized/test.compile.pass.cpp
+++ /dev/null
@@ -1,39 +0,0 @@
-// Copyright (c) Microsoft Corporation.
-// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
-
-#include <concepts>
-#include <memory>
-#include <memory_resource>
-#include <ostream>
-#include <sstream>
-#include <stacktrace>
-#include <string>
-
-using namespace std;
-
-template <class Ostream, class Alloc = allocator<stacktrace_entry>>
-concept CanPrintStacktrace = requires(Ostream& os, const stacktrace_entry& f, const basic_stacktrace<Alloc>& st) {
-                                 os << f;
-                                 os << st;
-                             };
-
-template <class CharT>
-struct FancyCharTraits : char_traits<CharT> {};
-
-static_assert(CanPrintStacktrace<ostream>);
-static_assert(CanPrintStacktrace<ostringstream>);
-static_assert(CanPrintStacktrace<ostream, pmr::polymorphic_allocator<stacktrace_entry>>);
-
-static_assert(!CanPrintStacktrace<wostream>);
-static_assert(!CanPrintStacktrace<wostringstream>);
-static_assert(!CanPrintStacktrace<wostream, pmr::polymorphic_allocator<stacktrace_entry>>);
-
-using FancyCharStream = basic_ostream<char, FancyCharTraits<char>>;
-static_assert(!CanPrintStacktrace<FancyCharStream>);
-static_assert(!CanPrintStacktrace<FancyCharStream, pmr::polymorphic_allocator<stacktrace_entry>>);
-
-using FancyWcharStream = basic_ostream<wchar_t, FancyCharTraits<wchar_t>>;
-static_assert(!CanPrintStacktrace<FancyWcharStream>);
-static_assert(!CanPrintStacktrace<FancyWcharStream, pmr::polymorphic_allocator<stacktrace_entry>>);
-
-int main() {} // COMPILE-ONLY
diff --git a/tests/std/tests/P0881R7_stacktrace/test.cpp b/tests/std/tests/P0881R7_stacktrace/test.cpp
index 74e67bf330..b726bb7dad 100644
--- a/tests/std/tests/P0881R7_stacktrace/test.cpp
+++ b/tests/std/tests/P0881R7_stacktrace/test.cpp
@@ -2,9 +2,12 @@
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 
 #include <cassert>
+#include <concepts>
 #include <cstddef>
 #include <filesystem>
 #include <iterator>
+#include <memory_resource>
+#include <ostream>
 #include <sstream>
 #include <stacktrace>
 #include <stdexcept>
@@ -20,6 +23,34 @@
 
 using namespace std;
 
+#if defined(__cpp_lib_concepts) // TRANSITION, GH-395
+template <class Ostream, class Alloc = allocator<stacktrace_entry>>
+concept CanPrintStacktrace =
+    requires(Ostream& os, const stacktrace_entry& f, const basic_stacktrace<Alloc>& st) {
+        { os << f } -> same_as<basic_ostream<typename Ostream::char_type, typename Ostream::traits_type>&>;
+        { os << st } -> same_as<basic_ostream<typename Ostream::char_type, typename Ostream::traits_type>&>;
+    };
+
+template <class CharT>
+struct FancyCharTraits : char_traits<CharT> {};
+
+static_assert(CanPrintStacktrace<ostream>);
+static_assert(CanPrintStacktrace<ostringstream>);
+static_assert(CanPrintStacktrace<ostream, pmr::polymorphic_allocator<stacktrace_entry>>);
+
+static_assert(!CanPrintStacktrace<wostream>);
+static_assert(!CanPrintStacktrace<wostringstream>);
+static_assert(!CanPrintStacktrace<wostream, pmr::polymorphic_allocator<stacktrace_entry>>);
+
+using FancyCharStream = basic_ostream<char, FancyCharTraits<char>>;
+static_assert(!CanPrintStacktrace<FancyCharStream>);
+static_assert(!CanPrintStacktrace<FancyCharStream, pmr::polymorphic_allocator<stacktrace_entry>>);
+
+using FancyWcharStream = basic_ostream<wchar_t, FancyCharTraits<wchar_t>>;
+static_assert(!CanPrintStacktrace<FancyWcharStream>);
+static_assert(!CanPrintStacktrace<FancyWcharStream, pmr::polymorphic_allocator<stacktrace_entry>>);
+#endif // defined(__cpp_lib_concepts)
+
 [[maybe_unused]] const int base_line = __LINE__;
 
 // Note: the below assumes that tail call optimization is disabled, which is the case in /Od