ast.c: AST.of checks if a given method object is defined in C

[Bug #18178]

ast.c

@@ -213,6 +213,9 @@ ast_s_of(rb_execution_context_t *ec, VALUE module, VALUE body, VALUE keep_script
         else {
             iseq = rb_method_iseq(body);
         }
+        if (!iseq) {
+            rb_raise(rb_eArgError, "cannot get AST for method that is not defined in Ruby");
+        }
         if (rb_iseq_from_eval_p(iseq)) {
             rb_raise(rb_eArgError, "cannot get AST for method defined in eval");
         }

test/ruby/test_ast.rb

@@ -231,6 +231,11 @@ class TestAst < Test::Unit::TestCase
     assert_raise(ArgumentError) { RubyVM::AbstractSyntaxTree.of(method) }
   end
 
+  def test_of_c_method
+    c = Class.new { attr_reader :foo }
+    assert_raise(ArgumentError) { RubyVM::AbstractSyntaxTree.of(c.instance_method(:foo)) }
+  end
+
   def test_scope_local_variables
     node = RubyVM::AbstractSyntaxTree.parse("_x = 0")
     lv, _, body = *node.children