cgen: fix ref and deref when an interface is used as a function parameter (fix #19947) (#19966)

vlib/v/gen/c/fn.v

@@ -2318,6 +2318,10 @@ fn (mut g Gen) ref_or_deref_arg(arg ast.CallArg, expected_type ast.Type, lang as
 					g.write(')')
 					return
 				}
+			} else if arg_sym.kind == .interface_ && exp_sym.kind == .interface_ {
+				if exp_is_ptr && !arg_is_ptr {
+					g.write('&')
+				}
 			}
 		}
 	} else if arg_typ.has_flag(.shared_f) && !expected_type.has_flag(.shared_f) {

vlib/v/tests/fn_call_interface_args_test.v

@@ -0,0 +1,12 @@
+interface Foo {}
+
+fn has_interface_args(mut a Foo, b &Foo, c Foo) {
+	assert a == &Foo(1)
+	assert b == &Foo(1)
+	assert c == Foo(1)
+}
+
+fn test_fn_call_interface_args() {
+	mut arg := Foo(1)
+	has_interface_args(mut arg, arg, arg)
+}