[PATCH] libstdc++: Add noexcept to bad_expected_access members (LWG 4031)

[Jonathan Wakely]

Tested x86_64-linux.

-- >8 --

libstdc++-v3/ChangeLog:

        * include/std/expected (bad_expected_access<void>): Add noexcept
        to special member functions, as per LWG 4031.
        * testsuite/20_util/expected/bad.cc: Check for nothrow copy and
        move members.
---
 libstdc++-v3/include/std/expected              |  8 ++++----
 libstdc++-v3/testsuite/20_util/expected/bad.cc | 13 +++++++++++++
 2 files changed, 17 insertions(+), 4 deletions(-)

diff --git a/libstdc++-v3/include/std/expected 
b/libstdc++-v3/include/std/expected
index 2594cfe131c..3c52f7db01e 100644
--- a/libstdc++-v3/include/std/expected
+++ b/libstdc++-v3/include/std/expected
@@ -79,10 +79,10 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
     {
     protected:
       bad_expected_access() noexcept { }
-      bad_expected_access(const bad_expected_access&) = default;
-      bad_expected_access(bad_expected_access&&) = default;
-      bad_expected_access& operator=(const bad_expected_access&) = default;
-      bad_expected_access& operator=(bad_expected_access&&) = default;
+      bad_expected_access(const bad_expected_access&) noexcept = default;
+      bad_expected_access(bad_expected_access&&) noexcept = default;
+      bad_expected_access& operator=(const bad_expected_access&) noexcept = 
default;
+      bad_expected_access& operator=(bad_expected_access&&) noexcept = default;
       ~bad_expected_access() = default;
 
     public:
diff --git a/libstdc++-v3/testsuite/20_util/expected/bad.cc 
b/libstdc++-v3/testsuite/20_util/expected/bad.cc
index c629e149da5..7e227f904a0 100644
--- a/libstdc++-v3/testsuite/20_util/expected/bad.cc
+++ b/libstdc++-v3/testsuite/20_util/expected/bad.cc
@@ -12,3 +12,16 @@ test_pr105146()
 {
   std::bad_expected_access(E{});
 }
+
+void
+test_lwg4031()
+{
+  struct test_type : std::bad_expected_access<void> { };
+
+  static_assert( std::is_nothrow_default_constructible_v<test_type> );
+  // LWG 4031. bad_expected_access<void> member functions should be noexcept
+  static_assert( std::is_nothrow_copy_constructible_v<test_type> );
+  static_assert( std::is_nothrow_move_constructible_v<test_type> );
+  static_assert( std::is_nothrow_copy_assignable_v<test_type> );
+  static_assert( std::is_nothrow_move_assignable_v<test_type> );
+}
-- 
2.45.2