match on tuple subject not narrowed to Never for assert_never exhaustiveness

[asukaminato0721]

Describe the Bug

from typing import assert_never
class RQ: pass
class QQ: pass
class RA: pass
class QA: pass
def f(q: RQ | QQ, a: RA | QA):
    match q, a:
        case RQ(), RA(): return 1
        case RQ(), _: return 2      # wildcard: covers the rest of the RQ row
        case QQ(), QA(): return 3
        case QQ(), _: return 4      # wildcard: covers the rest of the QQ row
        case unreachable:
            assert_never(unreachable)

ref zubanls/zuban#487

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