"pyang --ietf" Crashes with "AttributeError: i_type_spec" in a very specific scenario

[Hubro]

Lately pyang --ietf has been crashing when parsing one of our larger YANG models. After some troubleshooting, I have isolated the issue to a small block and created a minimal example:

module inventory {
  yang-version 1.1;
  namespace "http://foo.com/inventory";
  prefix inventory;

  container inventory {
    list interface-link {
      list interface {
        key "id";

        leaf id {
          must "deref(../id)/../role != 'access'" {
            error-message "Oh no!";
          }
          type leafref {
            path "deref(../type)/../id";
          }
        }
      }
    }
  }
}

(Yes I realize the target of the leafrefs are now gone, but that is not relevant to the crash)

Save this as "example.yang", then run:

$ pyang --ietf example.yang

Output:

Traceback (most recent call last):
  File "/home/tomas/.pyenv/versions/my-example-project/bin/pyang", line 580, in <module>
    run()
  File "/home/tomas/.pyenv/versions/my-example-project/bin/pyang", line 433, in run
    ctx_validate_and_prune()
  File "/home/tomas/.pyenv/versions/my-example-project/bin/pyang", line 429, in ctx_validate_and_prune
    ctx.validate()
  File "/home/tomas/.pyenv/versions/3.8.6/envs/my-example-project/lib/python3.8/site-packages/pyang/context.py", line 348, in validate
    statements.validate_module(self, m)
  File "/home/tomas/.pyenv/versions/3.8.6/envs/my-example-project/lib/python3.8/site-packages/pyang/statements.py", line 414, in validate_module
    iterate(module, phase)
  File "/home/tomas/.pyenv/versions/3.8.6/envs/my-example-project/lib/python3.8/site-packages/pyang/statements.py", line 402, in iterate
    iterate(s, phase)
  File "/home/tomas/.pyenv/versions/3.8.6/envs/my-example-project/lib/python3.8/site-packages/pyang/statements.py", line 402, in iterate
    iterate(s, phase)
  File "/home/tomas/.pyenv/versions/3.8.6/envs/my-example-project/lib/python3.8/site-packages/pyang/statements.py", line 402, in iterate
    iterate(s, phase)
  [Previous line repeated 1 more time]
  File "/home/tomas/.pyenv/versions/3.8.6/envs/my-example-project/lib/python3.8/site-packages/pyang/statements.py", line 406, in iterate
    iterate(s, phase)
  File "/home/tomas/.pyenv/versions/3.8.6/envs/my-example-project/lib/python3.8/site-packages/pyang/statements.py", line 371, in iterate
    res = f(ctx, stmt)
  File "/home/tomas/.pyenv/versions/3.8.6/envs/my-example-project/lib/python3.8/site-packages/pyang/statements.py", line 233, in <lambda>
    ('reference_2', 'must'):lambda ctx, s:v_reference_must(ctx, s),
  File "/home/tomas/.pyenv/versions/3.8.6/envs/my-example-project/lib/python3.8/site-packages/pyang/statements.py", line 2173, in v_reference_must
    v_xpath(ctx, stmt)
  File "/home/tomas/.pyenv/versions/3.8.6/envs/my-example-project/lib/python3.8/site-packages/pyang/statements.py", line 2190, in v_xpath
    xpath.v_xpath(ctx, stmt, node)
  File "/home/tomas/.pyenv/versions/3.8.6/envs/my-example-project/lib/python3.8/site-packages/pyang/xpath.py", line 85, in v_xpath
    chk_xpath_expr(ctx, stmt.i_orig_module, stmt.pos, node, node, q, None)
  File "/home/tomas/.pyenv/versions/3.8.6/envs/my-example-project/lib/python3.8/site-packages/pyang/xpath.py", line 107, in chk_xpath_expr
    chk_xpath_expr(ctx, mod, pos, initial, node, q[2], None)
  File "/home/tomas/.pyenv/versions/3.8.6/envs/my-example-project/lib/python3.8/site-packages/pyang/xpath.py", line 97, in chk_xpath_expr
    chk_xpath_path(ctx, mod, pos, initial, node, q)
  File "/home/tomas/.pyenv/versions/3.8.6/envs/my-example-project/lib/python3.8/site-packages/pyang/xpath.py", line 232, in chk_xpath_path
    (rettype, tgt) = chk_xpath_function(ctx, mod, pos, initial,
  File "/home/tomas/.pyenv/versions/3.8.6/envs/my-example-project/lib/python3.8/site-packages/pyang/xpath.py", line 206, in chk_xpath_function
    not isinstance(type_.i_type_spec,
AttributeError: i_type_spec

The YANG model compiles successfully and works as expected, so it should not make pyang crash.

[Hubro]

I downgraded to pyang 2.5.2 and the crash disappeared, so the problem was definitely introduced in 2.5.3.