From 91e7a0ce34772c0f638dbd8d63c8eca3ae6a66b5 Mon Sep 17 00:00:00 2001
From: Casey Carter <Casey@Carter.net>
Date: Fri, 21 Aug 2020 08:45:44 -0700
Subject: [PATCH 1/3] promise types _must_ have unhandled_exception in C++20

.... even when `_CPPUNWIND` is not defined.
---
 stl/inc/experimental/generator | 6 ++++--
 1 file changed, 4 insertions(+), 2 deletions(-)

diff --git a/stl/inc/experimental/generator b/stl/inc/experimental/generator
index af54662c112..9ad1cb20937 100644
--- a/stl/inc/experimental/generator
+++ b/stl/inc/experimental/generator
@@ -55,11 +55,13 @@ namespace experimental {
                 return {};
             }
 
-#ifdef _CPPUNWIND
             void unhandled_exception() noexcept {
+#ifdef _CPPUNWIND
                 _Eptr = _STD current_exception();
-            }
+#else // ^^^ _CPPUNWIND / !_CPPUNWIND vvv
+                abort();
 #endif // _CPPUNWIND
+            }
 
             void _Rethrow_if_exception() {
 #ifdef _CPPUNWIND

From f4be83e505f76e84cebc1e558baa7e77a8909528 Mon Sep 17 00:00:00 2001
From: Casey Carter <Casey@Carter.net>
Date: Fri, 21 Aug 2020 09:20:50 -0700
Subject: [PATCH 2/3] VSO-967814 has been fixed

... but VSO-1172852 remains, so we must keep the `exception_ptr` and rethrowing machinery in place for now. Let's annotate all of this workaround machinery properly, and demonstrate how to elide it after the bug is fixed.

Also avoid defining `unhandled_exception` in legacy coroutine mode when `_CPPUNWIND` isn't defined; legacy coroutines deal with this case in a different way that we don't want to regress.
---
 stl/inc/experimental/generator | 21 +++++++++++++++++----
 1 file changed, 17 insertions(+), 4 deletions(-)

diff --git a/stl/inc/experimental/generator b/stl/inc/experimental/generator
index 9ad1cb20937..fd8eb755bbb 100644
--- a/stl/inc/experimental/generator
+++ b/stl/inc/experimental/generator
@@ -38,10 +38,11 @@ namespace experimental {
     struct generator {
         struct promise_type {
             _Ty const* _CurrentValue;
+#if 1 // TRANSITION, VSO-1172852
 #ifdef _CPPUNWIND
-            // TRANSITION, VSO-967814
             exception_ptr _Eptr;
 #endif // _CPPUNWIND
+#endif // TRANSITION, VSO-1172852
 
             auto get_return_object() {
                 return generator{*this};
@@ -55,14 +56,21 @@ namespace experimental {
                 return {};
             }
 
+#if defined(_CPPUNWIND) || defined(__cpp_impl_coroutine)
             void unhandled_exception() noexcept {
+#if 1 // TRANSITION, VSO-1172852
 #ifdef _CPPUNWIND
                 _Eptr = _STD current_exception();
 #else // ^^^ _CPPUNWIND / !_CPPUNWIND vvv
                 abort();
 #endif // _CPPUNWIND
+#else // ^^^ workaround / no workaround
+                throw;
+#endif // TRANSITION, VSO-1172852
             }
+#endif // defined(_CPPUNWIND) || defined(__cpp_impl_coroutine)
 
+#if 1 // TRANSITION, VSO-1172852
             void _Rethrow_if_exception() {
 #ifdef _CPPUNWIND
                 if (_Eptr) {
@@ -70,6 +78,7 @@ namespace experimental {
                 }
 #endif // _CPPUNWIND
             }
+#endif // TRANSITION, VSO-1172852
 
             auto yield_value(_Ty const& _Value) {
                 _CurrentValue = _STD addressof(_Value);
@@ -119,7 +128,11 @@ namespace experimental {
             iterator& operator++() {
                 _Coro.resume();
                 if (_Coro.done()) {
+#if 1 // TRANSITION, VSO-1172852
                     _STD exchange(_Coro, {}).promise()._Rethrow_if_exception();
+#else // ^^^ workaround / no workaround
+                    _Coro = nullptr;
+#endif // TRANSITION, VSO-1172852
                 }
 
                 return *this;
@@ -152,7 +165,9 @@ namespace experimental {
             if (_Coro) {
                 _Coro.resume();
                 if (_Coro.done()) {
+#if 1 // TRANSITION, VSO-1172852
                     _Coro.promise()._Rethrow_if_exception();
+#endif // TRANSITION, VSO-1172852
                     return {nullptr};
                 }
             }
@@ -166,10 +181,8 @@ namespace experimental {
 
         explicit generator(promise_type& _Prom) : _Coro(coroutine_handle<promise_type>::from_promise(_Prom)) {}
 
-        generator() = default;
-
+        generator()                 = default;
         generator(generator const&) = delete;
-
         generator& operator=(generator const&) = delete;
 
         generator(generator&& _Right) : _Coro(_Right._Coro) {

From 268f3f83506b2827478d7b4cf047ef07e15d3ea3 Mon Sep 17 00:00:00 2001
From: Casey Carter <cacarter@microsoft.com>
Date: Mon, 24 Aug 2020 14:27:08 -0700
Subject: [PATCH 3/3] Apply suggestions from code review

Fixup #else comments.
---
 stl/inc/experimental/generator | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/stl/inc/experimental/generator b/stl/inc/experimental/generator
index fd8eb755bbb..60e2a896de7 100644
--- a/stl/inc/experimental/generator
+++ b/stl/inc/experimental/generator
@@ -64,7 +64,7 @@ namespace experimental {
 #else // ^^^ _CPPUNWIND / !_CPPUNWIND vvv
                 abort();
 #endif // _CPPUNWIND
-#else // ^^^ workaround / no workaround
+#else // ^^^ workaround / no workaround vvv
                 throw;
 #endif // TRANSITION, VSO-1172852
             }
@@ -130,7 +130,7 @@ namespace experimental {
                 if (_Coro.done()) {
 #if 1 // TRANSITION, VSO-1172852
                     _STD exchange(_Coro, {}).promise()._Rethrow_if_exception();
-#else // ^^^ workaround / no workaround
+#else // ^^^ workaround / no workaround vvv
                     _Coro = nullptr;
 #endif // TRANSITION, VSO-1172852
                 }