[Agda] Puzzled about how instance arguments work in the presence of metas

[Jesper Cockx]

Can someone tell me if the code fragment below is supposed to work? It
gives me yellow, but it works fine when I replace `it` by the (unique)
solution `is-zero`.

it : {A : Set} {{_ : A}} → A
it {{x}} = x

data IsZero : Nat → Set where
  instance
    is-zero : IsZero 0

test : IsZero _
test = it

Cheers,
Jesper
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