libstdc++: Implement LWG 3877 for std::expected monadic ops

This was approved in Issaquah 2023. As well as fixing the value
categories, this fixes the fact that we were incorrectly testing E
instead of T in the or_else constraints.

libstdc++-v3/ChangeLog:

	* include/std/expected (expected::and_then, expected::or_else)
	(expected::transform, expected::transform_error): Fix exception
	specifications as per LWG 3877.
	(expected<void, E>::and_then, expected<void, E>::transform):
	Likewise.
	* testsuite/20_util/expected/lwg3877.cc: New test.

2 files changed, 88 insertions, 24 deletions

diff --git a/libstdc++-v3/include/std/expected b/libstdc++-v3/include/std/expected
index c6d26b0d224..5ea0d6a7cb9 100644
--- a/libstdc++-v3/include/std/expected
+++ b/libstdc++-v3/include/std/expected
@@ -840,7 +840,7 @@ namespace __expected
 
       // monadic operations
 
-      template<typename _Fn> requires is_copy_constructible_v<_Er>
+      template<typename _Fn> requires is_constructible_v<_Er, _Er&>
 	constexpr auto
 	and_then(_Fn&& __f) &
 	{
@@ -854,7 +854,7 @@ namespace __expected
 	    return _Up(unexpect, error());
 	}
 
-      template<typename _Fn> requires is_copy_constructible_v<_Er>
+      template<typename _Fn> requires is_constructible_v<_Er, const _Er&>
 	constexpr auto
 	and_then(_Fn&& __f) const &
 	{
@@ -868,7 +868,7 @@ namespace __expected
 	    return _Up(unexpect, error());
 	}
 
-      template<typename _Fn> requires is_move_constructible_v<_Er>
+      template<typename _Fn> requires is_constructible_v<_Er, _Er>
 	constexpr auto
 	and_then(_Fn&& __f) &&
 	{
@@ -883,7 +883,7 @@ namespace __expected
 	}
 
 
-      template<typename _Fn> requires is_move_constructible_v<_Er>
+      template<typename _Fn> requires is_constructible_v<_Er, const _Er>
 	constexpr auto
 	and_then(_Fn&& __f) const &&
 	{
@@ -897,7 +897,7 @@ namespace __expected
 	    return _Up(unexpect, std::move(error()));
 	}
 
-      template<typename _Fn> requires is_copy_constructible_v<_Er>
+      template<typename _Fn> requires is_constructible_v<_Tp, _Tp&>
 	constexpr auto
 	or_else(_Fn&& __f) &
 	{
@@ -911,7 +911,7 @@ namespace __expected
 	    return std::__invoke(std::forward<_Fn>(__f), error());
 	}
 
-      template<typename _Fn> requires is_copy_constructible_v<_Er>
+      template<typename _Fn> requires is_constructible_v<_Tp, const _Tp&>
 	constexpr auto
 	or_else(_Fn&& __f) const &
 	{
@@ -926,7 +926,7 @@ namespace __expected
 	}
 
 
-      template<typename _Fn> requires is_move_constructible_v<_Er>
+      template<typename _Fn> requires is_constructible_v<_Tp, _Tp>
 	constexpr auto
 	or_else(_Fn&& __f) &&
 	{
@@ -940,7 +940,7 @@ namespace __expected
 	    return std::__invoke(std::forward<_Fn>(__f), std::move(error()));
 	}
 
-      template<typename _Fn> requires is_move_constructible_v<_Er>
+      template<typename _Fn> requires is_constructible_v<_Tp, const _Tp>
 	constexpr auto
 	or_else(_Fn&& __f) const &&
 	{
@@ -954,7 +954,7 @@ namespace __expected
 	    return std::__invoke(std::forward<_Fn>(__f), std::move(error()));
 	}
 
-      template<typename _Fn> requires is_copy_constructible_v<_Er>
+      template<typename _Fn> requires is_constructible_v<_Er, _Er&>
 	constexpr auto
 	transform(_Fn&& __f) &
 	{
@@ -970,7 +970,7 @@ namespace __expected
 	    return _Res(unexpect, std::move(error()));
 	}
 
-      template<typename _Fn> requires is_copy_constructible_v<_Er>
+      template<typename _Fn> requires is_constructible_v<_Er, const _Er&>
 	constexpr auto
 	transform(_Fn&& __f) const &
 	{
@@ -986,7 +986,7 @@ namespace __expected
 	    return _Res(unexpect, std::move(error()));
 	}
 
-      template<typename _Fn> requires is_move_constructible_v<_Er>
+      template<typename _Fn> requires is_constructible_v<_Er, _Er>
 	constexpr auto
 	transform(_Fn&& __f) &&
 	{
@@ -1002,7 +1002,7 @@ namespace __expected
 	    return _Res(unexpect, std::move(error()));
 	}
 
-      template<typename _Fn> requires is_move_constructible_v<_Er>
+      template<typename _Fn> requires is_constructible_v<_Er, const _Er>
 	constexpr auto
 	transform(_Fn&& __f) const &&
 	{
@@ -1018,7 +1018,7 @@ namespace __expected
 	    return _Res(unexpect, std::move(error()));
 	}
 
-      template<typename _Fn> requires is_copy_constructible_v<_Tp>
+      template<typename _Fn> requires is_constructible_v<_Tp, _Tp&>
 	constexpr auto
 	transform_error(_Fn&& __f) &
 	{
@@ -1034,7 +1034,7 @@ namespace __expected
 			});
 	}
 
-      template<typename _Fn> requires is_copy_constructible_v<_Tp>
+      template<typename _Fn> requires is_constructible_v<_Tp, const _Tp&>
 	constexpr auto
 	transform_error(_Fn&& __f) const &
 	{
@@ -1050,7 +1050,7 @@ namespace __expected
 			});
 	}
 
-      template<typename _Fn> requires is_move_constructible_v<_Tp>
+      template<typename _Fn> requires is_constructible_v<_Tp, _Tp>
 	constexpr auto
 	transform_error(_Fn&& __f) &&
 	{
@@ -1066,7 +1066,7 @@ namespace __expected
 			});
 	}
 
-      template<typename _Fn> requires is_move_constructible_v<_Tp>
+      template<typename _Fn> requires is_constructible_v<_Tp, const _Tp>
 	constexpr auto
 	transform_error(_Fn&& __f) const &&
 	{
@@ -1519,7 +1519,7 @@ namespace __expected
 
       // monadic operations
 
-      template<typename _Fn> requires is_copy_constructible_v<_Er>
+      template<typename _Fn> requires is_constructible_v<_Er, _Er&>
 	constexpr auto
 	and_then(_Fn&& __f) &
 	{
@@ -1533,7 +1533,7 @@ namespace __expected
 	    return _Up(unexpect, error());
 	}
 
-     template<typename _Fn> requires is_copy_constructible_v<_Er>
+     template<typename _Fn> requires is_constructible_v<_Er, const _Er&>
 	constexpr auto
 	and_then(_Fn&& __f) const &
 	{
@@ -1547,7 +1547,7 @@ namespace __expected
 	    return _Up(unexpect, error());
 	}
 
-      template<typename _Fn> requires is_move_constructible_v<_Er>
+      template<typename _Fn> requires is_constructible_v<_Er, _Er>
 	constexpr auto
 	and_then(_Fn&& __f) &&
 	{
@@ -1561,7 +1561,7 @@ namespace __expected
 	    return _Up(unexpect, std::move(error()));
 	}
 
-       template<typename _Fn> requires is_move_constructible_v<_Er>
+       template<typename _Fn> requires is_constructible_v<_Er, const _Er>
 	constexpr auto
 	and_then(_Fn&& __f) const &&
 	{
@@ -1631,7 +1631,7 @@ namespace __expected
 	    return std::__invoke(std::forward<_Fn>(__f), std::move(error()));
 	}
 
-      template<typename _Fn> requires is_copy_constructible_v<_Er>
+      template<typename _Fn> requires is_constructible_v<_Er, _Er&>
 	constexpr auto
 	transform(_Fn&& __f) &
 	{
@@ -1644,7 +1644,7 @@ namespace __expected
 	    return _Res(unexpect, error());
 	}
 
-      template<typename _Fn> requires is_copy_constructible_v<_Er>
+      template<typename _Fn> requires is_constructible_v<_Er, const _Er&>
 	constexpr auto
 	transform(_Fn&& __f) const &
 	{
@@ -1657,7 +1657,7 @@ namespace __expected
 	    return _Res(unexpect, error());
 	}
 
-      template<typename _Fn> requires is_move_constructible_v<_Er>
+      template<typename _Fn> requires is_constructible_v<_Er, _Er>
 	constexpr auto
 	transform(_Fn&& __f) &&
 	{
@@ -1670,7 +1670,7 @@ namespace __expected
 	    return _Res(unexpect, std::move(error()));
 	}
 
-      template<typename _Fn> requires is_move_constructible_v<_Er>
+      template<typename _Fn> requires is_constructible_v<_Er, const _Er>
 	constexpr auto
 	transform(_Fn&& __f) const &&
 	{
diff --git a/libstdc++-v3/testsuite/20_util/expected/lwg3877.cc b/libstdc++-v3/testsuite/20_util/expected/lwg3877.cc
new file mode 100644
index 00000000000..556d8d5dc9e
--- /dev/null
+++ b/libstdc++-v3/testsuite/20_util/expected/lwg3877.cc
@@ -0,0 +1,64 @@
+// { dg-options "-std=gnu++23" }
+// { dg-do compile { target c++23 } }
+
+#include <expected>
+
+struct T1
+{
+};
+
+struct T2
+{
+  T2(const T2&) = delete;
+};
+
+struct T3
+{
+  T3(const T3&) = delete;
+  T3(T3&&) = delete;
+  T3(T3&) { }
+  T3(const T3&&) { }
+};
+
+template<typename Exp, typename F>
+concept Has_or_else = requires(Exp&& exp, F f) {
+  std::forward<Exp>(exp).or_else(f);
+};
+
+using E1 = std::expected<T1, int>;
+static_assert( Has_or_else<E1&, E1(int)> );
+static_assert( Has_or_else<const E1&, E1(int)> );
+static_assert( Has_or_else<E1&&, E1(int)> );
+static_assert( Has_or_else<const E1&&, E1(int)> );
+
+using E2 = std::expected<T2, int>;
+static_assert( !Has_or_else<E2&, E2(int)> );
+static_assert( !Has_or_else<const E2&, E2(int)> );
+static_assert( !Has_or_else<E2&&, E2(int)> );
+static_assert( !Has_or_else<const E2&&, E2(int)> );
+
+using E3 = std::expected<T3, int>;
+static_assert( Has_or_else<E3&, E3(int)> );
+static_assert( !Has_or_else<const E3&, E3(int)> );
+static_assert( Has_or_else<E3&&, E3(int)> ); // uses or_else(F) const &&
+static_assert( Has_or_else<const E3&&, E3(int)> );
+
+template<typename Exp, typename F>
+concept Has_transform_error = requires(Exp&& exp, F f) {
+  std::forward<Exp>(exp).transform_error(f);
+};
+
+static_assert( Has_transform_error<E1&, int(int)> );
+static_assert( Has_transform_error<const E1&, int(int)> );
+static_assert( Has_transform_error<E1&&, int(int)> );
+static_assert( Has_transform_error<const E1&&, int(int)> );
+
+static_assert( !Has_transform_error<E2&, int(int)> );
+static_assert( !Has_transform_error<const E2&, int(int)> );
+static_assert( !Has_transform_error<E2&&, int(int)> );
+static_assert( !Has_transform_error<const E2&&, int(int)> );
+
+static_assert( Has_transform_error<E3&, int(int)> );
+static_assert( !Has_transform_error<const E3&, int(int)> );
+static_assert( Has_transform_error<E3&&, int(int)> ); // uses transform_error(F) const &&
+static_assert( Has_transform_error<const E3&&, int(int)> );