L 6a7bed94d3

fix: kernel exception from fvars left from ?m a b instantiation (#4410) ...

Closes #4375

The following example raises `error: (kernel) declaration has free
variables '_example'`:
```lean
example: Nat → Nat :=
  let a : Nat := Nat.zero
  fun (_ : Nat) =>
    let b : Nat := Nat.zero
    (fun (_ : a = b) => 0) (Eq.refl a)
```

During elaboration of `0`, `elabNumLit` creates a synthetic mvar
`?_uniq.16` which gets abstracted by `elabFun` to `?_uniq.16 :=
?_uniq.50 _uniq.6 _uniq.12`. The `isDefEq` to `instOfNatNat 0` results
in:
```
?_uniq.50 :=
  fun (x._@.4375._hyg.13 : Nat) =>
    let b : Nat := Nat.zero
    fun (x._@.4375._hyg.23 : Eq.{1} Nat _uniq.4 b) =>
      instOfNatNat 0
```

This has a free variable `_uniq.4` which was `a`.

When the application of `?_uniq.50` to `#[#2, #0]` is instantiated, the
`let b : Nat := Nat.zero` blocks the beta-reduction and `_uniq.4`
remains in the expression.

fix: add `(useZeta := true)` here:

ea46bf2839/src/Lean/MetavarContext.lean (L567)

2024-06-10 19:02:27 +00:00

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