On Wed, Sep 11, 2024 at 01:19:53PM -0400, Jason Merrill wrote:
> On 9/11/24 12:54 PM, Marek Polacek wrote:
> > +     auto_diagnostic_group d;
> > +     /* We used to emit a hard error, so this uses 0 rather than
> > +        OPT_Wpedantic.  */
> > +     if (pedwarn (DECL_SOURCE_LOCATION (fn), 0,
> > +                  "defaulted declaration %q+D does not match the "
> > +                  "expected signature", fn))
> > +       inform (DECL_SOURCE_LOCATION (fn),
> > +               "expected signature: %qD", implicit_fn);
> 
> This should also depend on -Wdefaulted-function-deleted, and set
> DECL_DELETED_FN.  And the C++20 case should show the expected signature.
> Really, the two cases should share the same code, only the diagnostic kind
> should change.

How about this?

dg.exp passed; running the full testing.

-- >8 --
This PR points out the we're not implementing [dcl.fct.def.default]
properly.  Consider e.g.

  struct C {
     C(const C&&) = default;
  };

where we wrongly emit an error, but the move ctor should be just =deleted.
According to [dcl.fct.def.default], if the type of the special member
function differs from the type of the corresponding special member function
that would have been implicitly declared in a way other than as allowed
by 2.1-4, the function is defined as deleted.  There's an exception for
assignment operators in which case the program is ill-formed.

clang++ has a warning for when we delete an explicitly-defaulted function
so this patch adds it too.  I'm also downgrading an error to a pedwarn
in C++17 since the code compiles in C++20.

        PR c++/116162

gcc/c-family/ChangeLog:

        * c.opt (Wdefaulted-function-deleted): New.

gcc/cp/ChangeLog:

        * class.cc (check_bases_and_members): Call delete_defaulted_fn to set
        DECL_DELETED_FN.
        * cp-tree.h (delete_defaulted_fn): Declare.
        * method.cc (delete_defaulted_fn): New.
        (defaulted_late_check): Call delete_defaulted_fn instead of giving
        an error.

gcc/ChangeLog:

        * doc/invoke.texi: Document -Wdefaulted-function-deleted.

gcc/testsuite/ChangeLog:

        * g++.dg/cpp0x/defaulted15.C: Add dg-warning/dg-error.
        * g++.dg/cpp0x/defaulted51.C: Likewise.
        * g++.dg/cpp0x/defaulted52.C: Likewise.
        * g++.dg/cpp0x/defaulted53.C: Likewise.
        * g++.dg/cpp0x/defaulted54.C: Likewise.
        * g++.dg/cpp0x/defaulted56.C: Likewise.
        * g++.dg/cpp0x/defaulted57.C: Likewise.
        * g++.dg/cpp0x/defaulted58.C: Likewise.
        * g++.dg/cpp0x/defaulted59.C: Likewise.
        * g++.dg/cpp0x/defaulted63.C: New test.
        * g++.dg/cpp0x/defaulted64.C: New test.
        * g++.dg/cpp0x/defaulted65.C: New test.
        * g++.dg/cpp23/defaulted1.C: New test.
---
 gcc/c-family/c.opt                       |  4 ++
 gcc/cp/class.cc                          | 18 ++++--
 gcc/cp/cp-tree.h                         |  1 +
 gcc/cp/method.cc                         | 77 +++++++++++++++++++++---
 gcc/doc/invoke.texi                      |  9 +++
 gcc/testsuite/g++.dg/cpp0x/defaulted15.C |  3 +-
 gcc/testsuite/g++.dg/cpp0x/defaulted51.C |  3 +-
 gcc/testsuite/g++.dg/cpp0x/defaulted52.C |  3 +-
 gcc/testsuite/g++.dg/cpp0x/defaulted53.C |  3 +-
 gcc/testsuite/g++.dg/cpp0x/defaulted54.C |  2 +
 gcc/testsuite/g++.dg/cpp0x/defaulted56.C |  6 +-
 gcc/testsuite/g++.dg/cpp0x/defaulted57.C |  6 +-
 gcc/testsuite/g++.dg/cpp0x/defaulted58.C |  2 +
 gcc/testsuite/g++.dg/cpp0x/defaulted59.C |  3 +-
 gcc/testsuite/g++.dg/cpp0x/defaulted63.C | 39 ++++++++++++
 gcc/testsuite/g++.dg/cpp0x/defaulted64.C | 27 +++++++++
 gcc/testsuite/g++.dg/cpp0x/defaulted65.C | 25 ++++++++
 gcc/testsuite/g++.dg/cpp23/defaulted1.C  | 23 +++++++
 18 files changed, 232 insertions(+), 22 deletions(-)
 create mode 100644 gcc/testsuite/g++.dg/cpp0x/defaulted63.C
 create mode 100644 gcc/testsuite/g++.dg/cpp0x/defaulted64.C
 create mode 100644 gcc/testsuite/g++.dg/cpp0x/defaulted65.C
 create mode 100644 gcc/testsuite/g++.dg/cpp23/defaulted1.C

diff --git a/gcc/c-family/c.opt b/gcc/c-family/c.opt
index 491aa02e1a3..f5136fd2341 100644
--- a/gcc/c-family/c.opt
+++ b/gcc/c-family/c.opt
@@ -619,6 +619,10 @@ Wdeclaration-missing-parameter-type
 C ObjC Var(warn_declaration_missing_parameter) Warning Init(1)
 Warn for missing parameter types in function declarations.
 
+Wdefaulted-function-deleted
+C++ ObjC++ Var(warn_defaulted_fn_deleted) Init(1) Warning
+Warn when an explicitly defaulted function is deleted.
+
 Wdelete-incomplete
 C++ ObjC++ Var(warn_delete_incomplete) Init(1) Warning
 Warn when deleting a pointer to incomplete type.
diff --git a/gcc/cp/class.cc b/gcc/cp/class.cc
index 950d83b0ea4..2d85681dc72 100644
--- a/gcc/cp/class.cc
+++ b/gcc/cp/class.cc
@@ -6490,8 +6490,9 @@ check_bases_and_members (tree t)
        && !DECL_ARTIFICIAL (fn)
        && DECL_DEFAULTED_IN_CLASS_P (fn))
       {
+       special_function_kind kind = special_function_p (fn);
        /* ...except handle comparisons later, in finish_struct_1.  */
-       if (special_function_p (fn) == sfk_comparison)
+       if (kind == sfk_comparison)
          continue;
 
        int copy = copy_fn_p (fn);
@@ -6503,10 +6504,17 @@ check_bases_and_members (tree t)
            bool fn_const_p = (copy == 2);
 
            if (fn_const_p && !imp_const_p)
-             /* If the function is defaulted outside the class, we just
-                give the synthesis error.  Core Issue #1331 says this is
-                no longer ill-formed, it is defined as deleted instead.  */
-             DECL_DELETED_FN (fn) = true;
+             {
+               tree implicit_fn
+                 = implicitly_declare_fn (kind, DECL_CONTEXT (fn),
+                                          /*const_p=*/false,
+                                          /*pattern_fn=*/NULL_TREE,
+                                          /*inherited_parms=*/NULL_TREE);
+               /* If the function is defaulted outside the class, we just
+                  give the synthesis error.  Core Issue #1331 says this is
+                  no longer ill-formed, it is defined as deleted instead.  */
+               maybe_delete_defaulted_fn (fn, implicit_fn);
+             }
          }
        defaulted_late_check (fn);
       }
diff --git a/gcc/cp/cp-tree.h b/gcc/cp/cp-tree.h
index 7baa2ccbe1e..5f3ac601f12 100644
--- a/gcc/cp/cp-tree.h
+++ b/gcc/cp/cp-tree.h
@@ -6929,6 +6929,7 @@ extern bool type_build_ctor_call          (tree);
 extern bool type_build_dtor_call               (tree);
 extern void explain_non_literal_class          (tree);
 extern void inherit_targ_abi_tags              (tree);
+extern void maybe_delete_defaulted_fn          (tree, tree);
 extern void defaulted_late_check               (tree);
 extern bool defaultable_fn_check               (tree);
 extern void check_abi_tags                     (tree);
diff --git a/gcc/cp/method.cc b/gcc/cp/method.cc
index 68a776d2c5a..46335ea2f37 100644
--- a/gcc/cp/method.cc
+++ b/gcc/cp/method.cc
@@ -3503,6 +3503,72 @@ implicitly_declare_fn (special_function_kind kind, tree 
type,
   return fn;
 }
 
+/* Mark an explicitly defaulted function FN as =deleted and warn.
+   IMPLICIT_FN is the corresponding special member function that
+   would have been implicitly declared.  */
+
+void
+maybe_delete_defaulted_fn (tree fn, tree implicit_fn)
+{
+  if (DECL_ARTIFICIAL (fn) || !DECL_DEFAULTED_IN_CLASS_P (fn))
+    return;
+
+  DECL_DELETED_FN (fn) = true;
+
+  if (!warn_defaulted_fn_deleted)
+    return;
+
+  auto_diagnostic_group d;
+  const special_function_kind kind = special_function_p (fn);
+  tree parmtype
+    = TREE_VALUE (DECL_XOBJ_MEMBER_FUNCTION_P (fn)
+                 ? TREE_CHAIN (TYPE_ARG_TYPES (TREE_TYPE (fn)))
+                 : FUNCTION_FIRST_USER_PARMTYPE (fn));
+  const bool pedwarn_p
+   = (cxx_dialect < cxx20
+      /* [dcl.fct.def.default] "if F1 is an assignment operator"...  */
+      || (SFK_ASSIGN_P (kind)
+         /* "and the return type of F1 differs from the return type of F2"  */
+         && (!same_type_p (TREE_TYPE (TREE_TYPE (fn)),
+                           TREE_TYPE (TREE_TYPE (implicit_fn)))
+             /* "or F1's non-object parameter type is not a reference,
+                the program is ill-formed"  */
+             || !TYPE_REF_P (parmtype))));
+
+  const char *wmsg;
+  switch (kind)
+    {
+    case sfk_copy_constructor:
+      wmsg = G_("explicitly defaulted copy constructor is implicitly deleted "
+               "because its declared type does not match the type of an "
+               "implicit copy constructor");
+      break;
+    case sfk_move_constructor:
+      wmsg = G_("explicitly defaulted move constructor is implicitly deleted "
+               "because its declared type does not match the type of an "
+               "implicit move constructor");
+      break;
+    case sfk_copy_assignment:
+      wmsg = G_("explicitly defaulted copy assignment operator is implicitly "
+               "deleted because its declared type does not match the type "
+               "of an implicit copy assignment operator");
+      break;
+    case sfk_move_assignment:
+      wmsg = G_("explicitly defaulted move assignment operator is implicitly "
+               "deleted because its declared type does not match the type "
+               "of an implicit move assignment operator");
+      break;
+    default:
+      gcc_unreachable ();
+    }
+  if (emit_diagnostic (pedwarn_p ? DK_PEDWARN : DK_WARNING,
+                      DECL_SOURCE_LOCATION (fn),
+                      OPT_Wdefaulted_function_deleted,
+                      wmsg))
+    inform (DECL_SOURCE_LOCATION (fn),
+           "expected signature: %qD", implicit_fn);
+}
+
 /* Gives any errors about defaulted functions which need to be deferred
    until the containing class is complete.  */
 
@@ -3529,7 +3595,8 @@ defaulted_late_check (tree fn)
 
   bool fn_const_p = (copy_fn_p (fn) == 2);
   tree implicit_fn = implicitly_declare_fn (kind, ctx, fn_const_p,
-                                           NULL, NULL);
+                                           /*pattern_fn=*/NULL_TREE,
+                                           /*inherited_parms=*/NULL_TREE);
   tree eh_spec = TYPE_RAISES_EXCEPTIONS (TREE_TYPE (implicit_fn));
 
   /* Includes special handling for a default xobj operator.  */
@@ -3558,13 +3625,7 @@ defaulted_late_check (tree fn)
   if (!same_type_p (TREE_TYPE (TREE_TYPE (fn)),
                    TREE_TYPE (TREE_TYPE (implicit_fn)))
       || !compare_fn_params (fn, implicit_fn))
-    {
-      auto_diagnostic_group d;
-      error ("defaulted declaration %q+D does not match the "
-            "expected signature", fn);
-      inform (DECL_SOURCE_LOCATION (fn),
-             "expected signature: %qD", implicit_fn);
-    }
+    maybe_delete_defaulted_fn (fn, implicit_fn);
 
   if (DECL_DELETED_FN (implicit_fn))
     {
diff --git a/gcc/doc/invoke.texi b/gcc/doc/invoke.texi
index b9a86a9a181..ca9e745f62e 100644
--- a/gcc/doc/invoke.texi
+++ b/gcc/doc/invoke.texi
@@ -251,6 +251,7 @@ in the following sections.
 -Wcomma-subscript  -Wconditionally-supported
 -Wno-conversion-null  -Wctad-maybe-unsupported
 -Wctor-dtor-privacy  -Wdangling-reference
+-Wno-defaulted-function-deleted
 -Wno-delete-incomplete
 -Wdelete-non-virtual-dtor  -Wno-deprecated-array-compare
 -Wdeprecated-copy -Wdeprecated-copy-dtor
@@ -4797,6 +4798,14 @@ not caught by reference. @option{-Wcatch-value} is 
enabled by @option{-Wall}.
 @item -Wconditionally-supported @r{(C++ and Objective-C++ only)}
 Warn for conditionally-supported (C++11 [intro.defs]) constructs.
 
+@opindex Wdefaulted-function-deleted
+@opindex Wno-defaulted-function-deleted
+@item -Wno-defaulted-function-deleted @r{(C++ and Objective-C++ only)}
+Warn when an explicitly defaulted function is deleted by the compiler.
+That can occur when the function's declared type does not match the type
+of the function that would have been implicitly declared.  This warning
+is enabled by default.
+
 @opindex Wdelete-incomplete
 @opindex Wno-delete-incomplete
 @item -Wno-delete-incomplete @r{(C++ and Objective-C++ only)}
diff --git a/gcc/testsuite/g++.dg/cpp0x/defaulted15.C 
b/gcc/testsuite/g++.dg/cpp0x/defaulted15.C
index 1e0b3545840..6bd02d74dc3 100644
--- a/gcc/testsuite/g++.dg/cpp0x/defaulted15.C
+++ b/gcc/testsuite/g++.dg/cpp0x/defaulted15.C
@@ -48,7 +48,8 @@ struct F
 
 struct G: public F
 {
-  G(const G&) = default;
+  G(const G&) = default;  // { dg-error "implicitly deleted" "" { target 
c++17_down } }
+                         // { dg-warning "implicitly deleted" "" { target 
c++20 } .-1 }
 };
 
 struct H
diff --git a/gcc/testsuite/g++.dg/cpp0x/defaulted51.C 
b/gcc/testsuite/g++.dg/cpp0x/defaulted51.C
index 0a7d308707c..561f8998177 100644
--- a/gcc/testsuite/g++.dg/cpp0x/defaulted51.C
+++ b/gcc/testsuite/g++.dg/cpp0x/defaulted51.C
@@ -4,7 +4,8 @@
 template<int> struct A
 {
   A();
-  A(volatile A&) = default;  // { dg-error "defaulted" }
+  A(volatile A&) = default;  // { dg-error "defaulted" "" { target c++17_down 
} }
+                            // { dg-warning "implicitly deleted" "" { target 
c++20 } .-1 }
 };
 
 struct B
diff --git a/gcc/testsuite/g++.dg/cpp0x/defaulted52.C 
b/gcc/testsuite/g++.dg/cpp0x/defaulted52.C
index c617230b493..9c10079362c 100644
--- a/gcc/testsuite/g++.dg/cpp0x/defaulted52.C
+++ b/gcc/testsuite/g++.dg/cpp0x/defaulted52.C
@@ -13,7 +13,8 @@ template<typename T> struct W
 {
   W();
   // This should now compile and be =deleted.
-  W(const W&) = default;
+  W(const W&) = default; // { dg-error "implicitly deleted" "" { target 
c++17_down } }
+// { dg-warning "implicitly deleted" "" { target c++20 } .-1 }
   T t;
 };
 
diff --git a/gcc/testsuite/g++.dg/cpp0x/defaulted53.C 
b/gcc/testsuite/g++.dg/cpp0x/defaulted53.C
index 8147e7e2ad1..cc039672277 100644
--- a/gcc/testsuite/g++.dg/cpp0x/defaulted53.C
+++ b/gcc/testsuite/g++.dg/cpp0x/defaulted53.C
@@ -14,7 +14,8 @@ struct R
 
 struct S
 {
-  S& operator=(const S&) = default;
+  S& operator=(const S&) = default; // { dg-error "implicitly deleted" "" { 
target c++17_down } }
+                                   // { dg-warning "implicitly deleted" "" { 
target c++20 } .-1 }
   M m;
 };
 
diff --git a/gcc/testsuite/g++.dg/cpp0x/defaulted54.C 
b/gcc/testsuite/g++.dg/cpp0x/defaulted54.C
index f8ddc4e47ce..2ca078acab5 100644
--- a/gcc/testsuite/g++.dg/cpp0x/defaulted54.C
+++ b/gcc/testsuite/g++.dg/cpp0x/defaulted54.C
@@ -11,6 +11,8 @@ template<typename T> struct W
 {
   W();
   W(const W&) = default; // { dg-error "binding" }
+// { dg-error "implicitly deleted" "" { target c++17_down } .-1 }
+// { dg-warning "implicitly deleted" "" { target c++20 } .-2 }
   T t;
 };
 
diff --git a/gcc/testsuite/g++.dg/cpp0x/defaulted56.C 
b/gcc/testsuite/g++.dg/cpp0x/defaulted56.C
index e7ce12c5566..0e36fd293f6 100644
--- a/gcc/testsuite/g++.dg/cpp0x/defaulted56.C
+++ b/gcc/testsuite/g++.dg/cpp0x/defaulted56.C
@@ -11,12 +11,14 @@ struct S
 
 struct T
 {
-  constexpr T(volatile T &) = default; // { dg-error "defaulted" }
+  constexpr T(volatile T &) = default; // { dg-error "defaulted" "" { target 
c++17_down } }
+                                      // { dg-warning "implicitly deleted" "" 
{ target c++20 } .-1 }
 };
 
 struct U
 {
-  constexpr U(const volatile U &) = default; // { dg-error "defaulted" }
+  constexpr U(const volatile U &) = default; // { dg-error "defaulted" "" { 
target c++17_down } }
+                                            // { dg-warning "implicitly 
deleted" "" { target c++20 } .-1 }
 };
 
 struct V
diff --git a/gcc/testsuite/g++.dg/cpp0x/defaulted57.C 
b/gcc/testsuite/g++.dg/cpp0x/defaulted57.C
index 37fb7dd6e1d..feca9662b4a 100644
--- a/gcc/testsuite/g++.dg/cpp0x/defaulted57.C
+++ b/gcc/testsuite/g++.dg/cpp0x/defaulted57.C
@@ -11,12 +11,14 @@ struct S
 
 struct T
 {
-  T& operator=(volatile T &) = default; // { dg-error "defaulted" }
+  T& operator=(volatile T &) = default; // { dg-error "defaulted" "" { target 
c++17_down } }
+                                       // { dg-warning "implicitly deleted" "" 
{ target c++20 } .-1 }
 };
 
 struct U
 {
-  U& operator=(const volatile U &) = default; // { dg-error "defaulted" }
+  U& operator=(const volatile U &) = default; // { dg-error "defaulted" "" { 
target c++17_down } }
+                                             // { dg-warning "implicitly 
deleted" "" { target c++20 } .-1 }
 };
 
 struct V
diff --git a/gcc/testsuite/g++.dg/cpp0x/defaulted58.C 
b/gcc/testsuite/g++.dg/cpp0x/defaulted58.C
index 920a4ad0c6d..8cb3e0effb9 100644
--- a/gcc/testsuite/g++.dg/cpp0x/defaulted58.C
+++ b/gcc/testsuite/g++.dg/cpp0x/defaulted58.C
@@ -11,6 +11,8 @@ template<typename T> struct W
 {
   W() = default;
   W& operator=(const W&) = default; // { dg-error "binding" }
+// { dg-error "implicitly deleted" "" { target c++17_down } .-1 }
+// { dg-warning "implicitly deleted" "" { target c++20 } .-2 }
   T t;
 };
 
diff --git a/gcc/testsuite/g++.dg/cpp0x/defaulted59.C 
b/gcc/testsuite/g++.dg/cpp0x/defaulted59.C
index 4f871d7f5b1..26510b4ee33 100644
--- a/gcc/testsuite/g++.dg/cpp0x/defaulted59.C
+++ b/gcc/testsuite/g++.dg/cpp0x/defaulted59.C
@@ -8,5 +8,6 @@ struct M
 
 struct W : public M
 {
-  W(const W&) = default;
+  W(const W&) = default; // { dg-error "implicitly deleted" "" { target 
c++17_down } }
+                        // { dg-warning "implicitly deleted" "" { target c++20 
} .-1 }
 };
diff --git a/gcc/testsuite/g++.dg/cpp0x/defaulted63.C 
b/gcc/testsuite/g++.dg/cpp0x/defaulted63.C
new file mode 100644
index 00000000000..99f92ff69c5
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp0x/defaulted63.C
@@ -0,0 +1,39 @@
+// PR c++/116162
+// { dg-do compile { target c++11 } }
+
+struct C0 {
+  C0(C0&) = default;
+};
+
+struct C1 {
+  C1(volatile C1&) = default; // { dg-warning "implicitly deleted" "" { target 
c++20 } }
+                             // { dg-error "does not match" "" { target 
c++17_down } .-1 }
+};
+
+struct C2 {
+  C2(const C2&) = default;
+};
+
+struct C3 {
+  C3(const volatile C3&) = default;  // { dg-warning "implicitly deleted" "" { 
target c++20 } }
+                                     // { dg-error "does not match" "" { 
target c++17_down } .-1 }
+};
+
+struct M0 {
+  M0(M0&&) = default;
+};
+
+struct M1 {
+  M1(const M1&&) = default; // { dg-warning "implicitly deleted" "" { target 
c++20 } }
+                           // { dg-error "does not match" "" { target 
c++17_down } .-1 }
+};
+
+struct M2 {
+  M2(volatile M2&&) = default; // { dg-warning "implicitly deleted" "" { 
target c++20 } }
+                               // { dg-error "does not match" "" { target 
c++17_down } .-1 }
+};
+
+struct M3 {
+  M3(const volatile M3&&) = default;  // { dg-warning "implicitly deleted" "" 
{ target c++20 } }
+                                     // { dg-error "does not match" "" { 
target c++17_down } .-1 }
+};
diff --git a/gcc/testsuite/g++.dg/cpp0x/defaulted64.C 
b/gcc/testsuite/g++.dg/cpp0x/defaulted64.C
new file mode 100644
index 00000000000..f20030192c3
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp0x/defaulted64.C
@@ -0,0 +1,27 @@
+// PR c++/116162
+// { dg-do compile { target c++11 } }
+
+struct M
+{
+  M& operator=(M&&);
+};
+
+struct R
+{
+  R& operator=(R&&) = default;
+  M m;
+};
+
+struct S
+{
+  S& operator=(const S&&) = default; // { dg-warning "implicitly deleted" "" { 
target c++20 } }
+                                    // { dg-error "does not match" "" { target 
c++17_down } .-1 }
+
+  M m;
+};
+
+struct T
+{
+  T operator=(T&&) = default; // { dg-error "defaulted" }
+  M m;
+};
diff --git a/gcc/testsuite/g++.dg/cpp0x/defaulted65.C 
b/gcc/testsuite/g++.dg/cpp0x/defaulted65.C
new file mode 100644
index 00000000000..88ca1d96084
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp0x/defaulted65.C
@@ -0,0 +1,25 @@
+// PR c++/116162
+// { dg-do compile { target c++11 } }
+
+struct S
+{
+  S& operator=(S &&) = default;
+};
+
+struct T
+{
+  T& operator=(volatile T &&) = default; // { dg-error "defaulted" "" { target 
c++17_down } }
+                                        // { dg-warning "implicitly deleted" 
"" { target c++20 } .-1 }
+};
+
+struct U
+{
+  U& operator=(const volatile U &&) = default; // { dg-error "defaulted" "" { 
target c++17_down } }
+                                              // { dg-warning "implicitly 
deleted" "" { target c++20 } .-1 }
+};
+
+struct V
+{
+  V& operator=(const V &&) = default; // { dg-error "defaulted" "" { target 
c++17_down } }
+                                     // { dg-warning "implicitly deleted" "" { 
target c++20 } .-1 }
+};
diff --git a/gcc/testsuite/g++.dg/cpp23/defaulted1.C 
b/gcc/testsuite/g++.dg/cpp23/defaulted1.C
new file mode 100644
index 00000000000..00cf894fa1d
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp23/defaulted1.C
@@ -0,0 +1,23 @@
+// PR c++/116162
+// { dg-do compile { target c++23 } }
+
+struct M
+{
+  M& operator=(M&);
+};
+
+struct T
+{
+  // if F1 is an assignment operator, and the return type of F1 differs
+  // from the return type,  the program is ill-formed.
+  T operator=(this T&, T&) = default; // { dg-error "defaulted" }
+  M m;
+};
+
+struct U
+{
+  // if F1's non-object parameter type is not a reference, the program
+  // is ill-formed.
+  U& operator=(this U&, U) = default; // { dg-error "defaulted" }
+  M m;
+};

base-commit: 670cfd5fe6433ee8f2e86eedb197d2523dbb033b
-- 
2.46.0

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