--disallow-any-generics is allowing defaultdict[Any, ...]

Bug Report

(A clear and concise description of what the bug is.)

To Reproduce

this python file (I was working on advent of code, this is about as minimal as I could reproduce)

import collections
import re

PATTERN = re.compile('^([^ ]+ [^ ]+) bags contain (.*)$')
BAG_RE = re.compile(r'(\d+) ([^ ]+ [^ ]+)')


def compute(s: str) -> int:
    parents = collections.defaultdict(list)
    for line in s.splitlines():
        match = PATTERN.match(line)
        assert match
        k = match[1]
        targets = [(int(n), tp) for n, tp in BAG_RE.findall(match[2])]
        for _, color in targets:
            parents[color].append(k)

    total_colors = set()
    todo = parents['shiny gold']
    while todo:
        color = todo.pop()
        if color not in total_colors:
            total_colors.add(color)
            todo.extend(parents[color])

    reveal_type(parents)

    return len(total_colors)

Run with mypy --disallow-any-generics t.py

Expected Behavior

I expect an error requiring me to annotate parents = collections.defaultdict(list) because mypy has not inferred the key type (though, I think it should be able to infer that it is str from the code)

Actual Behavior

$ mypy t.py --disallow-any-generics
t.py:26: note: Revealed type is 'collections.defaultdict[Any, builtins.list[builtins.str*]]'

Your Environment

• Mypy version used: 0.790
• Mypy command-line flags: --disallow-any-generics
• Mypy configuration options from mypy.ini (and other config files): N/A
• Python version used: 3.8.5
• Operating system and version: ubuntu 20.04

まず、t.py の再現コードに対して --disallow-any-generics を付けて mypy 0.790 を実行し、明示された defaultdict 型を確認します。collections.defaultdict(list) の処理とジェネリック型推論を追跡します。コマンドが意図した診断を報告するか、issue で期待されている動作と一貫したキー型を推論すれば完了です。