Exhaustiveness checking fails on non-trivial use of pattern matching

Code using assert_never() to ensure exhaustive use of match-case yields a type error on many non-trivial cases.

mypy can only verify exhaustiveness when matching directly against a Union type or Enum value.

Though, proving the exhaustiveness of non-trivial matches may be beyond mypy domain. Here are a couple of examples for the sake of argument:

Matching on both [] and [x, *xs] on Sequence will not narrow the type

from collections.abc import Sequence
from typing import assert_never

def my_sum(s: Sequence[float]) -> float:
    match s:
        case []:
            return 0
        case [num, *rest]:
            return num + my_sum(rest)
        case _ as u:
            # error: Argument 1 to "assert_never" has incompatible type "Sequence[float]"; expected "NoReturn"
            return assert_never(u)

Matching against all Enum values that a single-member wrapper class may contain

import dataclasses, enum
from typing import assert_never, final

class E(enum.Enum):
    A = enum.auto()
    B = enum.auto()

@final
@dataclasses.dataclass(frozen=True)
class Foo:
    x: E

def match_enum_attribute(f: Foo) -> None:
    match f:
        case Foo(E.A):
            pass
        case Foo(E.B):
            pass
        case _ as r:
            # error: Argument 1 to "assert_never" has incompatible type "Foo"; expected "NoReturn"  [arg-type]
            assert_never(r)

まず、assert_never()、match-case、Sequence パターン、および issue に示されている wrapped Enum 属性を使って 2 つの例を再現します。これらのケースに対する mypy の網羅性と narrowing の動作を追跡します。完了の条件は、サポートされている非自明な match が、報告された型エラーなしに網羅的であると認識され、例に対するカバレッジがあることです。