unknown free variable for mutual def with incrementality

Prerequisites

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Description

Code here:

mutual 
def a : Nat → Nat
| 0 => 1
| i + 1 => a i + b i
def b (i : Nat) : Nat := a i
def c (i : Nat) : Nat := b i
end

If you comment out the def c line, there's an error on the def a complaining about an unknown free variable. Uncommenting c fixes it, as does restarting file or just commenting and uncommenting the whole block.

Steps to Reproduce

1. Comment out the def c line, leaving a valid mutual def (as can be confirmed by restarting file) but that gives an error (for me, it's unknown free variable '_fvar.10').

Expected behavior: No error when removing c.

Actual behavior: Code errors when removing c.

Versions

On live.lean-lang.org on lean nightly:
Lean 4.23.0-nightly-2025-08-11
Target: x86_64-unknown-linux-gnu

Impact

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Reproduce the mutual definition example in Lean 4.23.0-nightly-2025-08-11, comparing the block with and without def c and after restarting or reloading it. Then trace the incremental processing of mutual definitions and free-variable checking. Done means removing c produces no unknown-free-variable error, matching a freshly processed file.