From 0fac85a24f40ef6098b756e8e8655205f4bfbf3e Mon Sep 17 00:00:00 2001
From: Jonathan Wakely <jwakely@redhat.com>
Date: Thu, 21 Oct 2021 01:19:45 +0100
Subject: [PATCH] libstdc++: Remove constraints from std::optional monadic ops
 [PR102863]

The constraints on transform and and_then can cause errors when checking
satisfaction. The constraints that were present in R6 of the paper were
moved for he final F8 revision, and so should have been included in the
implementation.

libstdc++-v3/ChangeLog:

	PR libstdc++/102863
	* include/std/optional (optional::and_then, optional::transform):
	Remove requires-clause.
	* testsuite/20_util/optional/monadic/and_then.cc: Check
	overload resolution doesn't cause errors.
	* testsuite/20_util/optional/monadic/transform.cc: Likewise.
---
 libstdc++-v3/include/std/optional                        | 16 ++++++++--------
 .../testsuite/20_util/optional/monadic/and_then.cc       | 12 ++++++++++++
 .../testsuite/20_util/optional/monadic/transform.cc      | 12 ++++++++++++
 3 files changed, 32 insertions(+), 8 deletions(-)

diff --git a/libstdc++-v3/include/std/optional b/libstdc++-v3/include/std/optional
index eac91d3..783d7ca 100644
--- a/libstdc++-v3/include/std/optional
+++ b/libstdc++-v3/include/std/optional
@@ -1048,7 +1048,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
 
       // [optional.monadic]
 
-      template<typename _Fn> requires invocable<_Fn, _Tp&>
+      template<typename _Fn>
 	constexpr auto
 	and_then(_Fn&& __f) &
 	{
@@ -1060,7 +1060,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
 	    return _Up();
 	}
 
-      template<typename _Fn> requires invocable<_Fn, const _Tp&>
+      template<typename _Fn>
 	constexpr auto
 	and_then(_Fn&& __f) const &
 	{
@@ -1072,7 +1072,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
 	    return _Up();
 	}
 
-      template<typename _Fn> requires invocable<_Fn, _Tp>
+      template<typename _Fn>
 	constexpr auto
 	and_then(_Fn&& __f) &&
 	{
@@ -1084,7 +1084,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
 	    return _Up();
 	}
 
-      template<typename _Fn> requires invocable<_Fn, const _Tp>
+      template<typename _Fn>
 	constexpr auto
 	and_then(_Fn&& __f) const &&
 	{
@@ -1096,7 +1096,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
 	    return _Up();
 	}
 
-      template<typename _Fn> requires invocable<_Fn, _Tp&>
+      template<typename _Fn>
 	constexpr auto
 	transform(_Fn&& __f) &
 	{
@@ -1107,7 +1107,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
 	    return optional<_Up>();
 	}
 
-      template<typename _Fn> requires invocable<_Fn, const _Tp&>
+      template<typename _Fn>
 	constexpr auto
 	transform(_Fn&& __f) const &
 	{
@@ -1118,7 +1118,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
 	    return optional<_Up>();
 	}
 
-      template<typename _Fn> requires invocable<_Fn, _Tp>
+      template<typename _Fn>
 	constexpr auto
 	transform(_Fn&& __f) &&
 	{
@@ -1129,7 +1129,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
 	    return optional<_Up>();
 	}
 
-      template<typename _Fn> requires invocable<_Fn, const _Tp>
+      template<typename _Fn>
 	constexpr auto
 	transform(_Fn&& __f) const &&
 	{
diff --git a/libstdc++-v3/testsuite/20_util/optional/monadic/and_then.cc b/libstdc++-v3/testsuite/20_util/optional/monadic/and_then.cc
index 02dcafe..f69ab95 100644
--- a/libstdc++-v3/testsuite/20_util/optional/monadic/and_then.cc
+++ b/libstdc++-v3/testsuite/20_util/optional/monadic/and_then.cc
@@ -113,8 +113,20 @@ test_forwarding()
 
 static_assert( test_forwarding() );
 
+void f(int&) { }
+
+void
+test_unconstrained()
+{
+  // PR libstc++/102863 - Optional monadic ops should not be constrained
+  std::optional<int> x;
+  auto answer = x.and_then([](auto& y) { f(y); return std::optional<int>{42}; });
+  VERIFY( !answer );
+}
+
 int main()
 {
   test_and_then();
   test_forwarding();
+  test_unconstrained();
 }
diff --git a/libstdc++-v3/testsuite/20_util/optional/monadic/transform.cc b/libstdc++-v3/testsuite/20_util/optional/monadic/transform.cc
index 13977b8..356c94d 100644
--- a/libstdc++-v3/testsuite/20_util/optional/monadic/transform.cc
+++ b/libstdc++-v3/testsuite/20_util/optional/monadic/transform.cc
@@ -132,9 +132,21 @@ test_copy_elision()
 
 static_assert( test_copy_elision() );
 
+void f(int&) { }
+
+void
+test_unconstrained()
+{
+  // PR libstc++/102863 - Optional monadic ops should not be constrained
+  std::optional<int> x;
+  auto answer = x.transform([](auto& y) { f(y); return 42; });
+  VERIFY( !answer );
+}
+
 int main()
 {
   test_transform();
   test_forwarding();
   test_copy_elision();
+  test_unconstrained();
 }
-- 
2.7.4