Merge pull request #5555 from dearblue/str-ref-regexp

Partially allow overriding of `String#[]` methods

src/vm.c

@@ -1261,8 +1261,16 @@ RETRY_TRY_BLOCK:
         regs[a] = mrb_hash_get(mrb, va, vb);
         break;
       case MRB_TT_STRING:
+        switch (mrb_type(vb)) {
+        case MRB_TT_INTEGER:
+        case MRB_TT_STRING:
+        case MRB_TT_RANGE:
           regs[a] = mrb_str_aref(mrb, va, vb, mrb_undef_value());
           break;
+        default:
+          goto getidx_fallback;
+        }
+        break;
       default:
       getidx_fallback:
         c = 1;