INTERNAL ERROR reported by mypy 0.610

[andymwood]

I'm getting an internal error when testing some code I'm trying to patch.

I've been unable to reproduce the problem with a trivial example, so I've pushed the changes to my GitHub profile.

To reproduce:

git clone [email protected]:andymwood/numpy-stubs.git
cd numpy-stubs
git checkout annotations       # Currently on commit 33496555
pip install .
mypy --strict tests/pass/simple.py

which produces the following output:

tests/pass/simple.py:76: error: INTERNAL ERROR -- please report a bug at https://github.com/python/mypy/issues version: 0.610
tests/pass/simple.py:76: : note: please use --show-traceback to print a traceback when reporting a bug

Here's the traceback:

Traceback (most recent call last):
  File "/home/andrew/.conda/envs/numpy-stubs/bin/mypy", line 11, in <module>
    sys.exit(console_entry())
  File "/home/andrew/.conda/envs/numpy-stubs/lib/python3.6/site-packages/mypy/__main__.py", line 7, in console_entry
    main(None)
  File "/home/andrew/.conda/envs/numpy-stubs/lib/python3.6/site-packages/mypy/main.py", line 91, in main
    res = type_check_only(sources, bin_dir, options, flush_errors, fscache)  # noqa
  File "/home/andrew/.conda/envs/numpy-stubs/lib/python3.6/site-packages/mypy/main.py", line 148, in type_check_only
    fscache=fscache)
  File "/home/andrew/.conda/envs/numpy-stubs/lib/python3.6/site-packages/mypy/build.py", line 185, in build
    flush_errors, fscache)
  File "/home/andrew/.conda/envs/numpy-stubs/lib/python3.6/site-packages/mypy/build.py", line 285, in _build
    graph = dispatch(sources, manager)
  File "/home/andrew/.conda/envs/numpy-stubs/lib/python3.6/site-packages/mypy/build.py", line 2398, in dispatch
    process_graph(graph, manager)
  File "/home/andrew/.conda/envs/numpy-stubs/lib/python3.6/site-packages/mypy/build.py", line 2691, in process_graph
    process_stale_scc(graph, scc, manager)
  File "/home/andrew/.conda/envs/numpy-stubs/lib/python3.6/site-packages/mypy/build.py", line 2807, in process_stale_scc
    graph[id].type_check_first_pass()
  File "/home/andrew/.conda/envs/numpy-stubs/lib/python3.6/site-packages/mypy/build.py", line 2029, in type_check_first_pass
    self.type_checker().check_first_pass()
  File "/home/andrew/.conda/envs/numpy-stubs/lib/python3.6/site-packages/mypy/checker.py", line 252, in check_first_pass
    self.accept(d)
  File "/home/andrew/.conda/envs/numpy-stubs/lib/python3.6/site-packages/mypy/checker.py", line 351, in accept
    stmt.accept(self)
  File "/home/andrew/.conda/envs/numpy-stubs/lib/python3.6/site-packages/mypy/nodes.py", line 883, in accept
    return visitor.visit_expression_stmt(self)
  File "/home/andrew/.conda/envs/numpy-stubs/lib/python3.6/site-packages/mypy/checker.py", line 2237, in visit_expression_stmt
    self.expr_checker.accept(s.expr, allow_none_return=True, always_allow_any=True)
  File "/home/andrew/.conda/envs/numpy-stubs/lib/python3.6/site-packages/mypy/checkexpr.py", line 2407, in accept
    typ = node.accept(self)
  File "/home/andrew/.conda/envs/numpy-stubs/lib/python3.6/site-packages/mypy/nodes.py", line 1543, in accept
    return visitor.visit_op_expr(self)
  File "/home/andrew/.conda/envs/numpy-stubs/lib/python3.6/site-packages/mypy/checkexpr.py", line 1396, in visit_op_expr
    allow_reverse=True)
  File "/home/andrew/.conda/envs/numpy-stubs/lib/python3.6/site-packages/mypy/checkexpr.py", line 1523, in check_op
    local_errors)
  File "/home/andrew/.conda/envs/numpy-stubs/lib/python3.6/site-packages/mypy/checkexpr.py", line 1507, in check_op_local
    callable_name=callable_name, object_type=object_type)
  File "/home/andrew/.conda/envs/numpy-stubs/lib/python3.6/site-packages/mypy/checkexpr.py", line 577, in check_call
    callee, args, arg_kinds, formal_to_actual, context)
  File "/home/andrew/.conda/envs/numpy-stubs/lib/python3.6/site-packages/mypy/checkexpr.py", line 829, in infer_function_type_arguments
    strict=self.chk.in_checked_function())
  File "/home/andrew/.conda/envs/numpy-stubs/lib/python3.6/site-packages/mypy/infer.py", line 31, in infer_function_type_arguments
    callee_type, arg_types, arg_kinds, formal_to_actual)
  File "/home/andrew/.conda/envs/numpy-stubs/lib/python3.6/site-packages/mypy/constraints.py", line 63, in infer_constraints_for_callable
    SUPERTYPE_OF)
  File "/home/andrew/.conda/envs/numpy-stubs/lib/python3.6/site-packages/mypy/constraints.py", line 170, in infer_constraints
    for t_item in template.items],
  File "/home/andrew/.conda/envs/numpy-stubs/lib/python3.6/site-packages/mypy/constraints.py", line 170, in <listcomp>
    for t_item in template.items],
  File "/home/andrew/.conda/envs/numpy-stubs/lib/python3.6/site-packages/mypy/constraints.py", line 190, in infer_constraints_if_possible
    return infer_constraints(template, actual, direction)
  File "/home/andrew/.conda/envs/numpy-stubs/lib/python3.6/site-packages/mypy/constraints.py", line 174, in infer_constraints
    return template.accept(ConstraintBuilderVisitor(actual, direction))
  File "/home/andrew/.conda/envs/numpy-stubs/lib/python3.6/site-packages/mypy/types.py", line 512, in accept
    return visitor.visit_instance(self)
  File "/home/andrew/.conda/envs/numpy-stubs/lib/python3.6/site-packages/mypy/constraints.py", line 351, in visit_instance
    original_actual, template)
  File "/home/andrew/.conda/envs/numpy-stubs/lib/python3.6/site-packages/mypy/constraints.py", line 393, in infer_constraints_from_protocol_members
    assert inst is not None and temp is not None
AssertionError:

[ilevkivskyi]

OK, I think I identified the cause: it is _promote. A simple repro is:

from typing import SupportsComplex, TypeVar, Union

T = TypeVar('T')

def f(x: Union[SupportsComplex, T]) -> Union[SupportsComplex, T]: ...

f(0)

For example in this particular case int doesn't implement SupportsComplex protocol, but it is considered a subtype of it because int is promoted to float, and then to complex. Then in the constraint inference code we assume that "if it is a subtype of a protocol, then it must have all members", which is not the case here.

A potential solution is to replace is_subtype with is_protocol_implementation in that protocol constraint inference branch.

[ilevkivskyi]

I actually think it is relatively important and the fix may be simple so I set priority to high.