Assertion error: `isinstance(ret, Instance)`

Crash Report

I tried running mypy on my codebase and it hit an INTERNAL ERROR.

Traceback

test_bloom_real.py:10: error: "AST" has no attribute "ops"  [attr-defined]
test_bloom_real.py:10: error: INTERNAL ERROR -- Please try using mypy master on GitHub:
https://mypy.readthedocs.io/en/stable/common_issues.html#using-a-development-mypy-build
Please report a bug at https://github.com/python/mypy/issues
version: 1.20.0+dev.131f9d92da58294bb2f273425e8778bd7d5b861f
Traceback (most recent call last):
  File "/home/danzin/venvs/wily_3.14_venv/bin/mypy", line 7, in <module>
    sys.exit(console_entry())
  File "/home/danzin/projects/mypy/mypy/__main__.py", line 15, in console_entry
    main()
  File "/home/danzin/projects/mypy/mypy/main.py", line 135, in main
    res, messages, blockers = run_build(sources, options, fscache, t0, stdout, stderr)
  File "/home/danzin/projects/mypy/mypy/main.py", line 219, in run_build
    res = build.build(sources, options, None, flush_errors, fscache, stdout, stderr)
  File "/home/danzin/projects/mypy/mypy/build.py", line 347, in build
    result = build_inner(
  File "/home/danzin/projects/mypy/mypy/build.py", line 444, in build_inner
    graph = dispatch(sources, manager, stdout)
  File "/home/danzin/projects/mypy/mypy/build.py", line 3212, in dispatch
    process_graph(graph, manager)
  File "/home/danzin/projects/mypy/mypy/build.py", line 3623, in process_graph
    done, still_working, results = manager.wait_for_done(graph)
  File "/home/danzin/projects/mypy/mypy/build.py", line 1097, in wait_for_done
    process_stale_scc(graph, next_scc, self)
  File "/home/danzin/projects/mypy/mypy/build.py", line 3787, in process_stale_scc
    graph[id].type_check_first_pass()
  File "/home/danzin/projects/mypy/mypy/build.py", line 2577, in type_check_first_pass
    self.type_checker().check_first_pass()
  File "/home/danzin/projects/mypy/mypy/checker.py", line 530, in check_first_pass
    self.accept(d)
  File "/home/danzin/projects/mypy/mypy/checker.py", line 641, in accept
    stmt.accept(self)
  File "/home/danzin/projects/mypy/mypy/nodes.py", line 1487, in accept
    return visitor.visit_class_def(self)
  File "/home/danzin/projects/mypy/mypy/checker.py", line 2687, in visit_class_def
    self.accept(defn.defs)
  File "/home/danzin/projects/mypy/mypy/checker.py", line 641, in accept
    stmt.accept(self)
  File "/home/danzin/projects/mypy/mypy/nodes.py", line 1582, in accept
    return visitor.visit_block(self)
  File "/home/danzin/projects/mypy/mypy/checker.py", line 3159, in visit_block
    self.accept(s)
  File "/home/danzin/projects/mypy/mypy/checker.py", line 641, in accept
    stmt.accept(self)
  File "/home/danzin/projects/mypy/mypy/nodes.py", line 1004, in accept
    return visitor.visit_func_def(self)
  File "/home/danzin/projects/mypy/mypy/checker.py", line 1198, in visit_func_def
    self._visit_func_def(defn)
  File "/home/danzin/projects/mypy/mypy/checker.py", line 1202, in _visit_func_def
    self.check_func_item(defn, name=defn.name)
  File "/home/danzin/projects/mypy/mypy/checker.py", line 1237, in check_func_item
    self.check_func_def(defn, typ, name, allow_empty)
  File "/home/danzin/projects/mypy/mypy/checker.py", line 1510, in check_func_def
    self.accept(item.body)
  File "/home/danzin/projects/mypy/mypy/checker.py", line 641, in accept
    stmt.accept(self)
  File "/home/danzin/projects/mypy/mypy/nodes.py", line 1582, in accept
    return visitor.visit_block(self)
  File "/home/danzin/projects/mypy/mypy/checker.py", line 3159, in visit_block
    self.accept(s)
  File "/home/danzin/projects/mypy/mypy/checker.py", line 641, in accept
    stmt.accept(self)
  File "/home/danzin/projects/mypy/mypy/nodes.py", line 1669, in accept
    return visitor.visit_assignment_stmt(self)
  File "/home/danzin/projects/mypy/mypy/checker.py", line 3210, in visit_assignment_stmt
    self.check_assignment(s.lvalues[-1], s.rvalue, s.type is None, s.new_syntax)
  File "/home/danzin/projects/mypy/mypy/checker.py", line 3436, in check_assignment
    rvalue_type = self.expr_checker.accept(rvalue, type_context=type_context)
  File "/home/danzin/projects/mypy/mypy/checkexpr.py", line 6087, in accept
    typ = node.accept(self)  # r-value type, when interpreted as a value expression
  File "/home/danzin/projects/mypy/mypy/nodes.py", line 2764, in accept
    return visitor.visit_list_comprehension(self)
  File "/home/danzin/projects/mypy/mypy/checkexpr.py", line 5784, in visit_list_comprehension
    return self.check_generator_or_comprehension(
  File "/home/danzin/projects/mypy/mypy/checkexpr.py", line 5845, in check_generator_or_comprehension
    return self.check_call(constructor, [gen.left_expr], [nodes.ARG_POS], gen)[0]
  File "/home/danzin/projects/mypy/mypy/checkexpr.py", line 1588, in check_call
    return self.check_callable_call(
  File "/home/danzin/projects/mypy/mypy/checkexpr.py", line 1767, in check_callable_call
    callee = self.infer_function_type_arguments(
  File "/home/danzin/projects/mypy/mypy/checkexpr.py", line 2092, in infer_function_type_arguments
    arg_types = self.infer_arg_types_in_context(
  File "/home/danzin/projects/mypy/mypy/checkexpr.py", line 1978, in infer_arg_types_in_context
    res.append(self.accept(arg, ctx))
  File "/home/danzin/projects/mypy/mypy/checkexpr.py", line 6085, in accept
    typ = self.accept_maybe_cache(node, type_context=type_context)
  File "/home/danzin/projects/mypy/mypy/checkexpr.py", line 6120, in accept_maybe_cache
    typ = node.accept(self)
  File "/home/danzin/projects/mypy/mypy/nodes.py", line 2298, in accept
    return visitor.visit_call_expr(self)
           ~~~~~~~~~~~~~~~~~~~~~~~^^^^^^
  File "/home/danzin/projects/mypy/mypy/checkexpr.py", line 508, in visit_call_expr
    return self.visit_call_expr_inner(e, allow_none_return=allow_none_return)
           ~~~~~~~~~~~~~~~~~~~~~~~~~~^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/home/danzin/projects/mypy/mypy/checkexpr.py", line 645, in visit_call_expr_inner
    ret_type = self.check_call_expr_with_callee_type(
        callee_type, e, fullname, object_type, member
    )
  File "/home/danzin/projects/mypy/mypy/checkexpr.py", line 1495, in check_call_expr_with_callee_type
    ret_type, callee_type = self.check_call(
                            ~~~~~~~~~~~~~~~^
        callee_type,
        ^^^^^^^^^^^^
    ...<6 lines>...
        object_type=object_type,
        ^^^^^^^^^^^^^^^^^^^^^^^^
    )
    ^
  File "/home/danzin/projects/mypy/mypy/checkexpr.py", line 1588, in check_call
    return self.check_callable_call(
           ~~~~~~~~~~~~~~~~~~~~~~~~^
        callee,
        ^^^^^^^
    ...<6 lines>...
        object_type,
        ^^^^^^^^^^^^
    )
    ^
  File "/home/danzin/projects/mypy/mypy/checkexpr.py", line 1692, in check_callable_call
    and callee.type_object().is_protocol
        ~~~~~~~~~~~~~~~~~~^^
  File "/home/danzin/projects/mypy/mypy/types.py", line 2348, in type_object
    assert isinstance(ret, Instance)
           ~~~~~~~~~~^^^^^^^^^^^^^^^
AssertionError:
test_bloom_real.py:10: : note: use --pdb to drop into pdb

To Reproduce

Just running mypy on the code below shows the issue, it also reproduces easily in the Playground.

import ast

class ComparisonSwapper(ast.NodeTransformer):
    OP_MAP = {
        ast.Lt: ast.GtE,
        ast.GtE: ast.Lt,
    }

    def visit_Compare(self, node: ast.AST) -> ast.AST:
        new_ops = [self.OP_MAP.get(type(op), type(op))() for op in node.ops]
        node.ops = new_ops
        return node

Gist URL: https://gist.github.com/mypy-play/dd0214051d0571c0ac920271a1ac4193
Playground URL: https://mypy-play.net/?mypy=latest&python=3.12&gist=dd0214051d0571c0ac920271a1ac4193

Your Environment

• Mypy version used: 1.20.0+dev.131f9d92da58294bb2f273425e8778bd7d5b861f
• Mypy command-line flags: --show-traceback
• Mypy configuration options from mypy.ini (and other config files): None
• Python version used: Python 3.15.0a3+ (heads/main-dirty:fca7fec88ce, Jan 13 2026, 12:23:26) [Clang 21.1.2 (2ubuntu6)] (also repros on 3.14 locally, 3.12 on the Playground etc.)
• Operating system and version: Ubuntu 25.10 (Questing Quokka) x86_64

Riproduci il crash con l’esempio ComparisonSwapper fornito, quindi segui il percorso di chiamata attraverso mypy/checkexpr.py fino a types.py, dove type_object() verifica tramite un’asserzione che il suo risultato sia un Instance. Indaga sul motivo per cui il percorso di chiamata relativo ai protocolli produce un altro tipo e aggiungi una copertura di regressione per l’esempio; il lavoro è completato quando mypy segnala l’errore di tipo sottostante senza un INTERNAL ERROR o un traceback di asserzione.