[Bug #19966] [PRISM] Fix singleton method definition

prism_compile.c

@@ -1346,7 +1346,13 @@ pm_compile_node(rb_iseq_t *iseq, const pm_node_t *node, LINK_ANCHOR *const ret,
         pm_scope_node_init((pm_node_t *)def_node, &next_scope_node, scope_node, parser);
         rb_iseq_t *method_iseq = NEW_ISEQ(next_scope_node, rb_id2str(method_name), ISEQ_TYPE_METHOD, lineno);
 
-        ADD_INSN2(ret, &dummy_line_node, definemethod, ID2SYM(method_name), method_iseq);
+        if (def_node->receiver) {
+            pm_compile_node(iseq, def_node->receiver, ret, src, false, scope_node);
+            ADD_INSN2(ret, &dummy_line_node, definesmethod, ID2SYM(method_name), method_iseq);
+        }
+        else {
+            ADD_INSN2(ret, &dummy_line_node, definemethod, ID2SYM(method_name), method_iseq);
+        }
         RB_OBJ_WRITTEN(iseq, Qundef, (VALUE)method_iseq);
 
         if (!popped) {

test/ruby/test_compile_prism.rb

@@ -409,6 +409,11 @@ module Prism
       test_prism_eval("alias :prism_a :to_s")
     end
 
+    def test_DefNode
+      test_prism_eval("def prism_method; end")
+      test_prism_eval("a = Object.new; def a.prism_singleton; :ok; end; a.prism_singleton")
+    end
+
     def test_UndefNode
       test_prism_eval("def prism_undef_node_1; end; undef prism_undef_node_1")
       test_prism_eval(<<-HERE