[gcc r12-10138] c++: defaulted op== for incomplete class [PR107291]

[gcc r12-10138] c++: defaulted op== for incomplete class [PR107291]

[Jason Merrill]

https://gcc.gnu.org/g:db8d5b0ad074344559b3201e567c1e47e65d0bdd

commit r12-10138-gdb8d5b0ad074344559b3201e567c1e47e65d0bdd
Author: Jason Merrill <jason@redhat.com>
Date:   Mon Feb 5 19:56:45 2024 -0500

    c++: defaulted op== for incomplete class [PR107291]
    
    After complaining about lack of friendship, we should not try to go on and
    define the defaulted comparison operator anyway.
    
            PR c++/107291
    
    gcc/cp/ChangeLog:
    
            * method.cc (early_check_defaulted_comparison): Fail if not friend.
    
    gcc/testsuite/ChangeLog:
    
            * g++.dg/cpp2a/spaceship-eq17.C: New test.
    
    (cherry picked from commit c5d34912ad576be1ef19be92f7eabde54b9089eb)

Diff:
---
 gcc/cp/method.cc                            | 6 +++++-
 gcc/testsuite/g++.dg/cpp2a/spaceship-eq17.C | 5 +++++
 2 files changed, 10 insertions(+), 1 deletion(-)

diff --git a/gcc/cp/method.cc b/gcc/cp/method.cc
index 903ee666ef39..be9406e33039 100644
--- a/gcc/cp/method.cc
+++ b/gcc/cp/method.cc
@@ -1222,7 +1222,11 @@ early_check_defaulted_comparison (tree fn)
 	  /* Defaulted outside the class body.  */
 	  ctx = TYPE_MAIN_VARIANT (parmtype);
 	  if (!is_friend (ctx, fn))
-	    error_at (loc, "defaulted %qD is not a friend of %qT", fn, ctx);
+	    {
+	      error_at (loc, "defaulted %qD is not a friend of %qT", fn, ctx);
+	      inform (location_of (ctx), "declared here");
+	      ok = false;
+	    }
 	}
       else if (!same_type_ignoring_top_level_qualifiers_p (parmtype, ctx))
 	saw_bad = true;
diff --git a/gcc/testsuite/g++.dg/cpp2a/spaceship-eq17.C b/gcc/testsuite/g++.dg/cpp2a/spaceship-eq17.C
new file mode 100644
index 000000000000..039bfac387c3
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp2a/spaceship-eq17.C
@@ -0,0 +1,5 @@
+// PR c++/107291
+// { dg-do compile { target c++20 } }
+
+struct S4;					   // { dg-message "declared here" }
+bool operator==(S4 const &, S4 const &) = default; // { dg-error "not a friend" }