[gcc r14-8752] libstdc++: Make std::function deduction guide support explicit object functions [PR113335]

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

https://gcc.gnu.org/g:6f75149488b74ae696788b8761d6fcfa754a0af7

commit r14-8752-g6f75149488b74ae696788b8761d6fcfa754a0af7
Author: Jonathan Wakely <jwakely@redhat.com>
Date:   Fri Feb 2 10:03:12 2024 +0000

    libstdc++: Make std::function deduction guide support explicit object functions [PR113335]
    
    This makes the deduction guides for std::function and std::packaged_task
    work for explicit object member functions, i.e. "deducing this", as per
    LWG 3617.
    
    libstdc++-v3/ChangeLog:
    
            PR libstdc++/113335
            * include/bits/std_function.h (__function_guide_helper): Add
            partial specialization for explicit object member functions, as
            per LWG 3617.
            * testsuite/20_util/function/cons/deduction_c++23.cc: Check
            explicit object member functions.
            * testsuite/30_threads/packaged_task/cons/deduction_c++23.cc:
            Likewise.

Diff:
---
 libstdc++-v3/include/bits/std_function.h           |  6 ++++
 .../20_util/function/cons/deduction_c++23.cc       | 35 ++++++++++++++++++++++
 .../packaged_task/cons/deduction_c++23.cc          | 35 ++++++++++++++++++++++
 3 files changed, 76 insertions(+)

diff --git a/libstdc++-v3/include/bits/std_function.h b/libstdc++-v3/include/bits/std_function.h
index 9130f9d28e30..79b59466fe9d 100644
--- a/libstdc++-v3/include/bits/std_function.h
+++ b/libstdc++-v3/include/bits/std_function.h
@@ -697,6 +697,12 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
     >
     { using type = _Res(_Args...); };
 
+#if __cpp_explicit_this_parameter >= 202110L
+  template<typename _Res, typename _Tp, bool _Nx, typename... _Args>
+    struct __function_guide_helper<_Res (*) (_Tp, _Args...) noexcept(_Nx)>
+    { using type = _Res(_Args...); };
+#endif
+
 #if __cpp_static_call_operator >= 202207L && __cpp_concepts >= 202002L
   template<typename _StaticCallOp>
     struct __function_guide_static_helper
diff --git a/libstdc++-v3/testsuite/20_util/function/cons/deduction_c++23.cc b/libstdc++-v3/testsuite/20_util/function/cons/deduction_c++23.cc
index 132bfb8fbfe4..8c70c1029296 100644
--- a/libstdc++-v3/testsuite/20_util/function/cons/deduction_c++23.cc
+++ b/libstdc++-v3/testsuite/20_util/function/cons/deduction_c++23.cc
@@ -21,3 +21,38 @@ test_static_call_operator()
   std::function func2 = F2{};
   check_type<std::function<float(char, void*)>>(func2);
 }
+
+void
+test_explicit_object_call_operator()
+{
+  // LWG 3617 - function/packaged_task deduction guides and deducing this
+  struct F {
+    int operator()(this const F&) { return 42; }
+  };
+
+  std::function g = F{};
+  check_type<std::function<int()>>(g);
+
+  struct F2 {
+    short operator()(this F2&, float&) { return 0; }
+  };
+
+  std::function g2 = F2{};
+  check_type<std::function<short(float&)>>(g2);
+
+  struct F3 {
+    void operator()(this const F3, char*, long) { }
+  };
+
+  std::function g3 = F3{};
+  check_type<std::function<void(char*, long)>>(g3);
+
+  struct F4 {
+    int i;
+    operator const int&() const { return i; }
+    const long& operator()(this int, const long& l) { return l; }
+  };
+
+  std::function g4 = F4{};
+  check_type<std::function<const long&(const long&)>>(g4);
+}
diff --git a/libstdc++-v3/testsuite/30_threads/packaged_task/cons/deduction_c++23.cc b/libstdc++-v3/testsuite/30_threads/packaged_task/cons/deduction_c++23.cc
index 6a43cf945d88..307522a73f5d 100644
--- a/libstdc++-v3/testsuite/30_threads/packaged_task/cons/deduction_c++23.cc
+++ b/libstdc++-v3/testsuite/30_threads/packaged_task/cons/deduction_c++23.cc
@@ -20,3 +20,38 @@ test_static_call_operator()
   std::packaged_task task2{ F2{} };
   check_type<std::packaged_task<float(char, void*)>>(task2);
 }
+
+void
+test_explicit_object_call_operator()
+{
+  // LWG 3617 - function/packaged_task deduction guides and deducing this
+  struct F {
+    int operator()(this const F&) { return 42; }
+  };
+
+  std::packaged_task g{ F{} };
+  check_type<std::packaged_task<int()>>(g);
+
+  struct F2 {
+    short operator()(this F2&, float&) { return 0; }
+  };
+
+  std::packaged_task g2{ F2{} };
+  check_type<std::packaged_task<short(float&)>>(g2);
+
+  struct F3 {
+    void operator()(this const F3, char*, long) { }
+  };
+
+  std::packaged_task g3{ F3{} };
+  check_type<std::packaged_task<void(char*, long)>>(g3);
+
+  struct F4 {
+    int i;
+    operator const int&() const { return i; }
+    const long& operator()(this int, const long& l) { return l; }
+  };
+
+  std::packaged_task g4{ F4{} };
+  check_type<std::packaged_task<const long&(const long&)>>(g4);
+}