This PR fixes a `(kernel) declaration has metavariables` error that
occurred when a `by` tactic was used in a dependent inductive type index
that refers to a previous index:
```lean
axiom P : Prop
axiom Q : P → Prop
-- Previously gave: (kernel) declaration has metavariables 'Foo'
inductive Foo : (h : P) → (Q (by exact h)) → Prop
```
The root cause: `elabDepArrow` calls `mkForallFVars [h_fvar] body`
before the `by` tactic's metavariable `?m` is resolved. Since `h_fvar`
is in `?m`'s local context, `elimMVarDeps` creates a delayed assignment
`?newMVar #[h_fvar] := ?m`. After `synthesizeSyntheticMVarsNoPostponing`
assigns `?m := h_fvar`, `instantiateMVars` can resolve the delayed
assignment (substituting `h_fvar` with the actual argument, `bvar 0`, in
the pending value), yielding the correct type `∀ (h : P), Q (bvar 0) →
Prop`. The fix is to call `instantiateMVars` on the header type right
after `synthesizeSyntheticMVarsNoPostponing` in `elabHeadersAux`.
Fixes#12543.
🤖 This PR was created with [Claude Code](https://claude.ai/claude-code).
Co-authored-by: Claude Sonnet 4.6 <noreply@anthropic.com>
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