[Bug #20468] Fix safe navigation in for variable

Author: nobu

compile.c

@@ -5380,12 +5380,17 @@ compile_massign_lhs(rb_iseq_t *iseq, LINK_ANCHOR *const pre, LINK_ANCHOR *const
 
         CHECK(COMPILE_POPPED(pre, "masgn lhs (NODE_ATTRASGN)", node));
 
+        bool safenav_call = false;
         LINK_ELEMENT *insn_element = LAST_ELEMENT(pre);
         iobj = (INSN *)get_prev_insn((INSN *)insn_element); /* send insn */
         ASSUME(iobj);
-        ELEM_REMOVE(LAST_ELEMENT(pre));
-        ELEM_REMOVE((LINK_ELEMENT *)iobj);
-        pre->last = iobj->link.prev;
+        ELEM_REMOVE(insn_element);
+        if (!IS_INSN_ID(iobj, send)) {
+            safenav_call = true;
+            iobj = (INSN *)get_prev_insn(iobj);
+            ELEM_INSERT_NEXT(&iobj->link, insn_element);
+        }
+        (pre->last = iobj->link.prev)->next = 0;
 
         const struct rb_callinfo *ci = (struct rb_callinfo *)OPERAND_AT(iobj, 0);
         int argc = vm_ci_argc(ci) + 1;
@@ -5404,7 +5409,9 @@ compile_massign_lhs(rb_iseq_t *iseq, LINK_ANCHOR *const pre, LINK_ANCHOR *const
             return COMPILE_NG;
         }
 
-        ADD_ELEM(lhs, (LINK_ELEMENT *)iobj);
+        iobj->link.prev = lhs->last;
+        lhs->last->next = &iobj->link;
+        for (lhs->last = &iobj->link; lhs->last->next; lhs->last = lhs->last->next);
         if (vm_ci_flag(ci) & VM_CALL_ARGS_SPLAT) {
             int argc = vm_ci_argc(ci);
             bool dupsplat = false;
@@ -5437,9 +5444,11 @@ compile_massign_lhs(rb_iseq_t *iseq, LINK_ANCHOR *const pre, LINK_ANCHOR *const
             }
             INSERT_BEFORE_INSN1(iobj, line_no, node_id, pushtoarray, INT2FIX(1));
         }
-        ADD_INSN(lhs, line_node, pop);
-        if (argc != 1) {
+        if (!safenav_call) {
             ADD_INSN(lhs, line_node, pop);
+            if (argc != 1) {
+                ADD_INSN(lhs, line_node, pop);
+            }
         }
         for (int i=0; i < argc; i++) {
             ADD_INSN(post, line_node, pop);

test/ruby/test_syntax.rb

@@ -1248,6 +1248,20 @@ def test_safe_call_in_massign_lhs
     assert_syntax_error("a&.x,=0", /multiple assignment destination/)
   end
 
+  def test_safe_call_in_for_variable
+    assert_valid_syntax("for x&.bar in []; end")
+    assert_separately([], "#{<<~"begin;"}\n#{<<~'end;'}")
+    begin;
+      foo = nil
+      for foo&.bar in [1]; end
+      assert_nil(foo)
+
+      foo = Struct.new(:bar).new
+      for foo&.bar in [1]; end
+      assert_equal(1, foo.bar)
+    end;
+  end
+
   def test_no_warning_logop_literal
     assert_warning("") do
       eval("true||raise;nil")