[gcc r12-4586] libstdc++: Remove constraints from std::optional monadic ops [PR102863]

[Jonathan Wakely <redi@gcc.gnu.org>]

https://gcc.gnu.org/g:0fac85a24f40ef6098b756e8e8655205f4bfbf3e

commit r12-4586-g0fac85a24f40ef6098b756e8e8655205f4bfbf3e
Author: Jonathan Wakely <jwakely@redhat.com>
Date:   Thu Oct 21 01:19:45 2021 +0100

    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.

Diff:
---
 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 eac91d3c160..783d7ca1b64 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 02dcafe6c58..f69ab952643 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 13977b8ba8d..356c94de6d0 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();
 }