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 --git a/gcc/cp/decl.cc b/gcc/cp/decl.cc
index 8468b778da8dc8afd3bcc0ebdfbfa28a4a73d1ed..301b38219424a264653bfa92a3eb74872dc4000d 100644 (file)
--- 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 (file)
index 0000000..bd1bac6
--- /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 (file)
index 0000000..d29a2fb
--- /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);
+}