From 8f6c9ebc7d7726596c94f164176b914801c5665c Mon Sep 17 00:00:00 2001
From: Jason Merrill <jason@redhat.com>
Date: Sat, 1 Nov 2025 21:56:26 +0300
Subject: [PATCH] c++: ADL finds all friends (P1787)

Some libstdc++ tests were failing with import std because ADL didn't find
rethrow_exception, even though it's a friend.  The problem turned out to be
because it's not in the same namespace, and in C++17 ADL only makes hidden
friends visible.  But in C++20 P1787 changed [basic.lookup.argdep]/4.2 to
directly include all friends in the lookup.

Note that my change still excludes class members, even though the standard
doesn't specify that; including implicit object member functions would just
break, and even foreign static/xobj member functions seem like they would be
strange to include.

gcc/cp/ChangeLog:

	* name-lookup.cc (name_lookup::adl_class_fns): Include all
	namespace-scope friends.

gcc/testsuite/ChangeLog:

	* g++.dg/lookup/koenig16.C: New test.
---
 gcc/cp/name-lookup.cc                   |  9 ++++++---
 gcc/testsuite/g++.dg/lookup/koenig16.C  | 22 ++++++++++++++++++++++
 gcc/testsuite/g++.dg/modules/adl-11_a.C | 21 +++++++++++++++++++++
 gcc/testsuite/g++.dg/modules/adl-11_b.C | 12 ++++++++++++
 4 files changed, 61 insertions(+), 3 deletions(-)
 create mode 100644 gcc/testsuite/g++.dg/lookup/koenig16.C
 create mode 100644 gcc/testsuite/g++.dg/modules/adl-11_a.C
 create mode 100644 gcc/testsuite/g++.dg/modules/adl-11_b.C

diff --git a/gcc/cp/name-lookup.cc b/gcc/cp/name-lookup.cc
index 2b34102c730..ef1360464c5 100644
--- a/gcc/cp/name-lookup.cc
+++ b/gcc/cp/name-lookup.cc
@@ -1372,11 +1372,14 @@ name_lookup::adl_class_fns (tree type)
 	  {
 	    tree fn = TREE_VALUE (friends);
 
-	    /* Only interested in global functions with potentially hidden
-	       (i.e. unqualified) declarations.  */
+	    /* Before C++20, ADL just makes hidden friends visible, so we
+	       only include functions in the same namespace.  After C++20,
+	       include all namespace-scope functions.  */
 	    if (!context)
 	      context = decl_namespace_context (type);
-	    if (CP_DECL_CONTEXT (fn) != context)
+	    if (cxx_dialect < cxx20
+		? CP_DECL_CONTEXT (fn) != context
+		: !DECL_NAMESPACE_SCOPE_P (fn))
 	      continue;
 
 	    dedup (true);
diff --git a/gcc/testsuite/g++.dg/lookup/koenig16.C b/gcc/testsuite/g++.dg/lookup/koenig16.C
new file mode 100644
index 00000000000..1d6e4e367eb
--- /dev/null
+++ b/gcc/testsuite/g++.dg/lookup/koenig16.C
@@ -0,0 +1,22 @@
+// Before P1787 (C++20), only hidden friends are included in ADL.
+// After P1787, all friends are included.
+
+namespace N {
+  namespace NN {
+    struct A;
+  }
+  using NN::A;
+  void fn (A);
+  namespace NN {
+    struct A {
+      friend void N::fn (A);
+    };
+  }
+  void fn (A) { }
+}
+
+int main()
+{
+  N::A a;
+  fn(a);			// { dg-error "not declared" "" { target c++17_down } }
+}
diff --git a/gcc/testsuite/g++.dg/modules/adl-11_a.C b/gcc/testsuite/g++.dg/modules/adl-11_a.C
new file mode 100644
index 00000000000..063dd89983a
--- /dev/null
+++ b/gcc/testsuite/g++.dg/modules/adl-11_a.C
@@ -0,0 +1,21 @@
+// Before P1787 (C++20), only hidden friends are included in ADL.
+// After P1787, all friends are included.
+
+// { dg-additional-options "-fmodules -Wno-global-module" }
+
+export module M;
+
+namespace N {
+  namespace NN {
+    export struct A;
+  }
+  export using NN::A;
+
+  export void fn (A);
+
+  namespace NN {
+    struct A {
+      friend void N::fn (A);
+    };
+  }
+}
diff --git a/gcc/testsuite/g++.dg/modules/adl-11_b.C b/gcc/testsuite/g++.dg/modules/adl-11_b.C
new file mode 100644
index 00000000000..f1789151834
--- /dev/null
+++ b/gcc/testsuite/g++.dg/modules/adl-11_b.C
@@ -0,0 +1,12 @@
+// Before P1787 (C++20), only hidden friends are included in ADL.
+// After P1787, all friends are included.
+
+// { dg-additional-options -fmodules }
+
+import M;
+
+int main()
+{
+  N::A a;
+  fn(a);			// { dg-error "not declared" "" { target c++17_down } }
+}
-- 
2.47.3