[gcc r13-5033] libstdc++: Reduce size of std::bind_front(empty_type) result [PR108290]

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

https://gcc.gnu.org/g:e2eab3c4edb6aa9a93f982c4554cd756000934ca

commit r13-5033-ge2eab3c4edb6aa9a93f982c4554cd756000934ca
Author: Jonathan Wakely <jwakely@redhat.com>
Date:   Thu Jan 5 11:35:35 2023 +0000

    libstdc++: Reduce size of std::bind_front(empty_type) result [PR108290]
    
    libstdc++-v3/ChangeLog:
    
            PR libstdc++/108290
            * include/std/functional (_Bind_front): Add no_unique_address
            attribute to data members.
            * testsuite/20_util/function_objects/bind_front/107784.cc: Check
            size of call wrappers with empty types for targets and bound
            arguments.

Diff:
---
 libstdc++-v3/include/std/functional                |  6 ++--
 .../20_util/function_objects/bind_front/107784.cc  | 38 +++++++++++++++++++++-
 2 files changed, 40 insertions(+), 4 deletions(-)

diff --git a/libstdc++-v3/include/std/functional b/libstdc++-v3/include/std/functional
index dddd22fcd04..5dff5be089d 100644
--- a/libstdc++-v3/include/std/functional
+++ b/libstdc++-v3/include/std/functional
@@ -991,8 +991,8 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
 	      std::forward<_CallArgs>(__call_args)...);
 	}
 
-      _Fd _M_fd;
-      std::tuple<_BoundArgs...> _M_bound_args;
+      [[no_unique_address]] _Fd _M_fd;
+      [[no_unique_address]] std::tuple<_BoundArgs...> _M_bound_args;
     };
 
   // Avoid the overhead of an empty tuple<> if there are no bound args.
@@ -1051,7 +1051,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
 	}
 
     private:
-      _Fd _M_fd;
+      [[no_unique_address]] _Fd _M_fd;
     };
 
   template<typename _Fn, typename... _Args>
diff --git a/libstdc++-v3/testsuite/20_util/function_objects/bind_front/107784.cc b/libstdc++-v3/testsuite/20_util/function_objects/bind_front/107784.cc
index ec255f3ee36..f1f8cee4509 100644
--- a/libstdc++-v3/testsuite/20_util/function_objects/bind_front/107784.cc
+++ b/libstdc++-v3/testsuite/20_util/function_objects/bind_front/107784.cc
@@ -10,6 +10,42 @@ struct Foo
 
 void bar() { }
 
-// PR libstdc++/107784
+// PR libstdc++/107784 - QOI: sizeof( bind_front( Member-Function ) ) too big
 static_assert( sizeof(std::bind_front(&Foo::func)) == sizeof(&Foo::func) );
 static_assert( sizeof(std::bind_front(&bar)) == sizeof(&bar) );
+
+// PR libstdc++/108290 - QoI: bind_front captureless lambda is too big
+auto empty_lambda = [](auto, auto) { return 0; };
+
+struct {
+  void operator()(int, int, int) { }
+  template<typename T> void operator()(T, T) { }
+} empty_class;
+
+static_assert(sizeof(std::bind_front(empty_lambda)) == 1);
+static_assert(sizeof(std::bind_front(empty_lambda, 1)) == sizeof(int));
+static_assert(sizeof(std::bind_front(empty_lambda, empty_lambda)) == 2);
+static_assert(sizeof(std::bind_front(empty_lambda, empty_class)) == 1);
+static_assert(sizeof(std::bind_front(empty_lambda, 1, 2)) == 2 * sizeof(int));
+static_assert(sizeof(std::bind_front(empty_lambda, '1', empty_lambda)) == 2);
+static_assert(sizeof(std::bind_front(empty_lambda, '1', empty_class)) == 1);
+
+static_assert(sizeof(std::bind_front(empty_class)) == 1);
+static_assert(sizeof(std::bind_front(empty_class, 1)) == sizeof(int));
+static_assert(sizeof(std::bind_front(empty_class, empty_lambda)) == 1);
+static_assert(sizeof(std::bind_front(empty_class, empty_class)) == 2);
+static_assert(sizeof(std::bind_front(empty_class, 1, 2)) == 2 * sizeof(int));
+static_assert(sizeof(std::bind_front(empty_class, '1', empty_lambda)) == 1);
+static_assert(sizeof(std::bind_front(empty_class, '1', empty_class)) == 2);
+
+struct derived1 : decltype(std::bind_front(empty_class))
+{
+  int i;
+};
+static_assert(sizeof(derived1) == sizeof(int));
+
+struct derived2 : decltype(std::bind_front(empty_class, empty_lambda))
+{
+  int i;
+};
+static_assert(sizeof(derived2) == sizeof(int));