Remove support for nil::Constant

This was an intentional bug added in 1.9.

The approach taken here is to add a second operand to the
getconstant instruction for whether nil should be allowed and
treated as current scope.

Fixes [Bug #11718]

compile.c

@@ -4404,18 +4404,18 @@ compile_const_prefix(rb_iseq_t *iseq, const NODE *const node,
     switch (nd_type(node)) {
       case NODE_CONST:
 	debugi("compile_const_prefix - colon", node->nd_vid);
-	ADD_INSN1(body, nd_line(node), getconstant, ID2SYM(node->nd_vid));
+	ADD_INSN2(body, nd_line(node), getconstant, ID2SYM(node->nd_vid), 1);
 	break;
       case NODE_COLON3:
 	debugi("compile_const_prefix - colon3", node->nd_mid);
 	ADD_INSN(body, nd_line(node), pop);
 	ADD_INSN1(body, nd_line(node), putobject, rb_cObject);
-	ADD_INSN1(body, nd_line(node), getconstant, ID2SYM(node->nd_mid));
+	ADD_INSN2(body, nd_line(node), getconstant, ID2SYM(node->nd_mid), 1);
 	break;
       case NODE_COLON2:
 	CHECK(compile_const_prefix(iseq, node->nd_head, pref, body));
 	debugi("compile_const_prefix - colon2", node->nd_mid);
-	ADD_INSN1(body, nd_line(node), getconstant, ID2SYM(node->nd_mid));
+	ADD_INSN2(body, nd_line(node), getconstant, ID2SYM(node->nd_mid), 0);
 	break;
       default:
 	CHECK(COMPILE(pref, "const colon2 prefix", node));
@@ -7154,7 +7154,7 @@ iseq_compile_each0(rb_iseq_t *iseq, LINK_ANCHOR *const ret, const NODE *node, in
 	    ADD_INSNL(ret, line, branchunless, lassign); /* cref */
 	}
 	ADD_INSN(ret, line, dup); /* cref cref */
-	ADD_INSN1(ret, line, getconstant, ID2SYM(mid)); /* cref obj */
+	ADD_INSN2(ret, line, getconstant, ID2SYM(mid), 1); /* cref obj */
 
 	if (node->nd_aid == idOROP || node->nd_aid == idANDOP) {
 	    lfin = NEW_LABEL(line);
@@ -7500,13 +7500,13 @@ iseq_compile_each0(rb_iseq_t *iseq, LINK_ANCHOR *const ret, const NODE *node, in
 	    int ic_index = body->is_size++;
 
             ADD_INSN2(ret, line, opt_getinlinecache, lend, INT2FIX(ic_index));
-	    ADD_INSN1(ret, line, getconstant, ID2SYM(node->nd_vid));
+	    ADD_INSN2(ret, line, getconstant, ID2SYM(node->nd_vid), 1);
             ADD_INSN1(ret, line, opt_setinlinecache, INT2FIX(ic_index));
 	    ADD_LABEL(ret, lend);
 	}
 	else {
 	    ADD_INSN(ret, line, putnil);
-	    ADD_INSN1(ret, line, getconstant, ID2SYM(node->nd_vid));
+	    ADD_INSN2(ret, line, getconstant, ID2SYM(node->nd_vid), 1);
 	}
 
 	if (popped) {
@@ -7893,7 +7893,7 @@ iseq_compile_each0(rb_iseq_t *iseq, LINK_ANCHOR *const ret, const NODE *node, in
 	}
 
 	ADD_INSN1(ret, line, putobject, rb_cObject);
-	ADD_INSN1(ret, line, getconstant, ID2SYM(node->nd_mid));
+	ADD_INSN2(ret, line, getconstant, ID2SYM(node->nd_mid), 1);
 
 	if (ISEQ_COMPILE_DATA(iseq)->option->inline_const_cache) {
             ADD_INSN1(ret, line, opt_setinlinecache, INT2FIX(ic_index));

insns.def

@@ -252,19 +252,19 @@ setclassvariable
     rb_cvar_set(vm_get_cvar_base(vm_get_cref(GET_EP()), GET_CFP()), id, val);
 }
 
-/* Get constant variable id. If klass is Qnil, constants
+/* Get constant variable id. If klass is Qnil and allow_nil is true, constants
    are searched in the current scope. Otherwise, get constant under klass
    class or module.
  */
 DEFINE_INSN
 getconstant
-(ID id)
+(ID id, VALUE allow_nil)
 (VALUE klass)
 (VALUE val)
 /* getconstant can kick autoload */
 // attr bool leaf = false; /* has rb_autoload_load() */
 {
-    val = vm_get_ev_const(ec, klass, id, 0);
+    val = vm_get_ev_const(ec, klass, id, allow_nil, 0);
 }
 
 /* Set constant variable id under cbase class or module.

test/ruby/test_const.rb

@@ -48,6 +48,12 @@ class TestConst < Test::Unit::TestCase
     assert_equal 8, TEST4
   end
 
+  def test_const_access_from_nil
+    assert_raise(TypeError) { eval("nil::Object") }
+    assert_raise(TypeError) { eval("c = nil; c::Object") }
+    assert_raise(TypeError) { eval("sc = Class.new; sc::C = nil; sc::C::Object") }
+  end
+
   def test_redefinition
     c = Class.new
     name = "X\u{5b9a 6570}"

vm_insnhelper.c

@@ -890,12 +890,12 @@ vm_get_iclass(rb_control_frame_t *cfp, VALUE klass)
 }
 
 static inline VALUE
-vm_get_ev_const(rb_execution_context_t *ec, VALUE orig_klass, ID id, int is_defined)
+vm_get_ev_const(rb_execution_context_t *ec, VALUE orig_klass, ID id, int allow_nil, int is_defined)
 {
     void rb_const_warn_if_deprecated(const rb_const_entry_t *ce, VALUE klass, ID id);
     VALUE val;
 
-    if (orig_klass == Qnil) {
+    if (orig_klass == Qnil && allow_nil) {
 	/* in current lexical scope */
         const rb_cref_t *root_cref = vm_get_cref(ec->cfp->ep);
 	const rb_cref_t *cref;
@@ -3146,7 +3146,7 @@ vm_defined(rb_execution_context_t *ec, rb_control_frame_t *reg_cfp, rb_num_t op_
       }
       case DEFINED_CONST:
 	klass = v;
-	if (vm_get_ev_const(ec, klass, SYM2ID(obj), 1)) {
+	if (vm_get_ev_const(ec, klass, SYM2ID(obj), 1, 1)) {
 	    expr_type = DEFINED_CONST;
 	}
 	break;