"incompatible default for argument" error message shows that types do match

[kamahen]

mypy --python-version=3.6 --strict-optional --check-untyped-defs --disallow-incomplete-defs --warn-incomplete-stub --warn-redundant-casts --warn-no-return --warn-return-any --no-incremental /tmp/xx.py 
/tmp/xx.py:24: error: Incompatible default for argument "_DISPATCH" (default has type "Dict[int, Callable[[Base, Ctx], AstNode]]", argument has type "Dict[int, Callable[[Base, Ctx], AstNode]]")

Note that the two types in the error message are identical.

When I did reveal_type(_DISPATCH), I got:

Revealed type is 'builtins.dict[builtins.int*, def (node: lib2to3.pytree.Base, ctx: xx.Ctx) -> xx.AstNode]'

Here's the source (I'm sure it can be reduced further):

from lib2to3 import pytree
from lib2to3.pgen2 import token
from typing import Callable, Dict, FrozenSet, List, Optional, Sequence, Text

class AstNode:
    pass


class Ctx:
    pass


def cvt_token_name(node: pytree.Base, ctx: Ctx) -> AstNode:
    """Handle token.NAME."""
    return AstNode()


_DISPATCH = {
    token.NAME: cvt_token_name,
}


def cvt(node: pytree.Base,
        ctx: Ctx,
        _DISPATCH: Dict[int, Callable[[pytree.Base, Ctx], AstNode]] = _DISPATCH
        ) -> AstNode:
    return _DISPATCH[node.type](node, ctx)

[JukkaL]

Here is a simplified repro:

from typing import List, Callable

def f(a: str) -> None: pass
l = [f]

def g(x: List[Callable[[str], None]]) -> None: pass
g(l)

The reason for the complaint is that the callable in l has the argument name (a) specified but the argument x of g has only an argument type, no name. The error message is very confusing and should indicate that the missing callable argument name is the issue (#4530).

Potential workarounds:

• Use a covariant container type in cvt / g, as suggested by mypy.
• Add an explicit type annotation for _DISPATCH / l.
• Include argument names in the function signature (using mypy_extensions.Arg).

Closing as duplicate of #4530.