[gcc r16-6366] c++: Fix member-like friend detection for non-template classes [PR122550]

[Patrick Palka]

https://gcc.gnu.org/g:3f09320bf65312a2f442aca514a35a382a6521f1

commit r16-6366-g3f09320bf65312a2f442aca514a35a382a6521f1
Author: Egas Ribeiro <egas.g.ribeiro@gmail.com>
Date:   Fri Dec 19 21:34:55 2025 +0000

    c++: Fix member-like friend detection for non-template classes [PR122550]
    
    member_like_constrained_friend_p was incorrectly returning true for
    constrained friend function templates declared in non-template classes,
    causing them to be treated as distinct from their forward declarations.
    This led to ambiguity errors at call sites.
    
    Per [temp.friend]/9, a constrained friend is only "member-like" (and thus
    declares a different function) in two cases:
    1. Non-template friends with constraints (must be in a templated class)
    2. Template friends whose constraints depend on outer template parameters
    
    In both cases, the enclosing class scope must be templated. The fix adds
    a check for CLASSTYPE_IMPLICIT_INSTANTIATION to ensure the friend's
    context is actually a class template, not a plain class or explicit
    specialization.
    
            PR c++/122550
    
    gcc/cp/ChangeLog:
    
            * decl.cc (member_like_constrained_friend_p): Check that the
            friend's enclosing class is an implicit instantiation.
    
    gcc/testsuite/ChangeLog:
    
            * g++.dg/cpp2a/concepts-friend18.C: New test.
            * g++.dg/cpp2a/concepts-friend18a.C: New test.
    
    Signed-off-by: Egas Ribeiro <egas.g.ribeiro@gmail.com>
    Reviewed-by: Patrick Palka <ppalka@redhat.com>

Diff:
---
 gcc/cp/decl.cc                                  |  1 +
 gcc/testsuite/g++.dg/cpp2a/concepts-friend18.C  | 19 +++++++++++++++++++
 gcc/testsuite/g++.dg/cpp2a/concepts-friend18a.C | 22 ++++++++++++++++++++++
 3 files changed, 42 insertions(+)

diff --git a/gcc/cp/decl.cc b/gcc/cp/decl.cc
index 8468b778da8d..301b38219424 100644
--- a/gcc/cp/decl.cc
+++ b/gcc/cp/decl.cc
@@ -1129,6 +1129,7 @@ member_like_constrained_friend_p (tree decl)
 	  && DECL_UNIQUE_FRIEND_P (decl)
 	  && DECL_FRIEND_CONTEXT (decl)
 	  && get_constraints (decl)
+	  && CLASSTYPE_IMPLICIT_INSTANTIATION (DECL_FRIEND_CONTEXT (decl))
 	  && (!DECL_TEMPLATE_INFO (decl)
 	      || !PRIMARY_TEMPLATE_P (DECL_TI_TEMPLATE (decl))
 	      || (uses_outer_template_parms_in_constraints
diff --git a/gcc/testsuite/g++.dg/cpp2a/concepts-friend18.C b/gcc/testsuite/g++.dg/cpp2a/concepts-friend18.C
new file mode 100644
index 000000000000..bd1bac6cdacd
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp2a/concepts-friend18.C
@@ -0,0 +1,19 @@
+// PR c++/122550
+// { dg-do compile { target c++20 } }
+
+struct Hasher;
+template <class a>
+concept C = true;
+
+template<C T>
+void add(Hasher&, T);
+
+struct Hasher {
+    template<C T>
+    friend void add(Hasher& hasher, T integer) {}
+};
+
+int main() {
+    Hasher h;
+    add(h, 0);
+}
diff --git a/gcc/testsuite/g++.dg/cpp2a/concepts-friend18a.C b/gcc/testsuite/g++.dg/cpp2a/concepts-friend18a.C
new file mode 100644
index 000000000000..d29a2fba0371
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp2a/concepts-friend18a.C
@@ -0,0 +1,22 @@
+// PR c++/122550
+// { dg-do compile { target c++20 } }
+
+template<typename U>
+struct Hasher;
+
+template <class a>
+concept C = true;
+
+template<C T>
+void add(Hasher<int>&, T);
+
+template<>
+struct Hasher<int> {
+    template<C T>
+    friend void add(Hasher& hasher, T integer) {}
+};
+
+int main() {
+    Hasher<int> h;
+    add(h, 0);
+}