[gcc r14-3902] libstdc++: Fix std::bind_front perfect forwarding [PR111327]

[Patrick Palka <ppalka@gcc.gnu.org>]

https://gcc.gnu.org/g:4289f6ceefe74ea46e792692448c06197ac20c86

commit r14-3902-g4289f6ceefe74ea46e792692448c06197ac20c86
Author: Patrick Palka <ppalka@redhat.com>
Date:   Tue Sep 12 11:23:12 2023 -0400

    libstdc++: Fix std::bind_front perfect forwarding [PR111327]
    
    In order to properly implement a perfect forwarding call wrapper (without
    C++23 deducing 'this') we need a total of 8 operator() overloads, 4
    enabled ones and 4 deleted ones, i.e. two for each const/ref qual pair,
    as described in section 5.5 of P0847R6.  Otherwise the wrapper may not
    do the right thing if the underlying function object has a deleted
    const/ref-qualified operator() overload.  This patch fixes this issue in
    std::bind_front.
    
            PR libstdc++/111327
    
    libstdc++-v3/ChangeLog:
    
            * include/std/functional (_Bind_front::operator()): Add deleted
            fallback overloads for each const/ref qualifier pair.  Give the
            enabled overloads dummy constraints to make each one more
            specialized than the corresponding deleted overload.
            * testsuite/20_util/function_objects/bind_front/111327.cc: New test.

Diff:
---
 libstdc++-v3/include/std/functional                | 16 +++++++++
 .../20_util/function_objects/bind_front/111327.cc  | 41 ++++++++++++++++++++++
 2 files changed, 57 insertions(+)

diff --git a/libstdc++-v3/include/std/functional b/libstdc++-v3/include/std/functional
index 7d1b890bb4e9..c50b9e4d3658 100644
--- a/libstdc++-v3/include/std/functional
+++ b/libstdc++-v3/include/std/functional
@@ -938,6 +938,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
       ~_Bind_front() = default;
 
       template<typename... _CallArgs>
+	requires true
 	constexpr
 	invoke_result_t<_Fd&, _BoundArgs&..., _CallArgs...>
 	operator()(_CallArgs&&... __call_args) &
@@ -948,6 +949,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
 	}
 
       template<typename... _CallArgs>
+	requires true
 	constexpr
 	invoke_result_t<const _Fd&, const _BoundArgs&..., _CallArgs...>
 	operator()(_CallArgs&&... __call_args) const &
@@ -959,6 +961,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
 	}
 
       template<typename... _CallArgs>
+	requires true
 	constexpr
 	invoke_result_t<_Fd, _BoundArgs..., _CallArgs...>
 	operator()(_CallArgs&&... __call_args) &&
@@ -969,6 +972,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
 	}
 
       template<typename... _CallArgs>
+	requires true
 	constexpr
 	invoke_result_t<const _Fd, const _BoundArgs..., _CallArgs...>
 	operator()(_CallArgs&&... __call_args) const &&
@@ -979,6 +983,18 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
 	      std::forward<_CallArgs>(__call_args)...);
 	}
 
+      template<typename... _CallArgs>
+	void operator()(_CallArgs&&...) & = delete;
+
+      template<typename... _CallArgs>
+	void operator()(_CallArgs&&...) const & = delete;
+
+      template<typename... _CallArgs>
+	void operator()(_CallArgs&&...) && = delete;
+
+      template<typename... _CallArgs>
+	void operator()(_CallArgs&&...) const && = delete;
+
     private:
       using _BoundIndices = index_sequence_for<_BoundArgs...>;
 
diff --git a/libstdc++-v3/testsuite/20_util/function_objects/bind_front/111327.cc b/libstdc++-v3/testsuite/20_util/function_objects/bind_front/111327.cc
new file mode 100644
index 000000000000..fcfb9f9c7c02
--- /dev/null
+++ b/libstdc++-v3/testsuite/20_util/function_objects/bind_front/111327.cc
@@ -0,0 +1,41 @@
+// PR libstdc++/111327 - std::bind_front (and std::not_fn) doesn't always
+// perfectly forward according to value category of the call wrapper object
+// { dg-options "-std=gnu++20" }
+// { dg-do compile { target c++20 } }
+
+#include <functional>
+#include <utility>
+
+struct F {
+  void operator()(...) & = delete;
+  void operator()(...) const &;
+};
+
+struct G {
+  void operator()(...) && = delete;
+  void operator()(...) const &&;
+};
+
+int main() {
+  auto f0 = std::bind_front(F{});
+  f0(); // { dg-error "deleted" }
+  std::move(f0)();
+  std::as_const(f0)();
+  std::move(std::as_const(f0))();
+
+  auto g0 = std::bind_front(G{});
+  g0(); // { dg-error "deleted" }
+  std::move(g0)(); // { dg-error "deleted" }
+  std::move(std::as_const(g0))();
+
+  auto f1 = std::bind_front(F{}, 42);
+  f1(); // { dg-error "deleted" }
+  std::move(f1)();
+  std::as_const(f1)();
+  std::move(std::as_const(f1))();
+
+  auto g1 = std::bind_front(G{}, 42);
+  g1(); // { dg-error "deleted" }
+  std::move(g1)(); // { dg-error "deleted" }
+  std::move(std::as_const(g1))();
+}