This PR fixes#12495 where equational theorem generation fails for
structurally recursive definitions using a Box-like wrapper around
nested inductives.
## Root Cause
`withInferTypeConfig` (in `InferType.lean`) ensures various MetaM config
settings (`beta`, `iota`, `zeta`, `zetaHave`, `zetaDelta`, `proj`) are
enabled during type inference, but was missing `etaStruct`. When
`inferType` is called from a context where `etaStruct` is disabled —
such as inside `simpMatch` (which sets `etaStruct := .none` via
`SimpM.run` → `withSimpContext`) — `whnf` cannot eta-expand structure
values needed for recursor iota reduction.
Concretely, projecting from a type like `Rec.rec_2 ... base` (where
`base : Box Rec`) requires eta-expanding `base` to `Box.mk base.data` so
the `Box` recursor can reduce. With `etaStruct := .none`,
`toCtorWhenStructure` skips the eta-expansion, leaving `whnf` stuck and
`inferProjType` unable to recognize the resulting type as a structure.
## Fix
Add `etaStruct := .all` to the config settings ensured by
`withInferTypeConfig`, alongside the existing `beta`, `iota`, `zeta`,
`zetaHave`, `zetaDelta`, and `proj` settings. This also allows reverting
the workaround (`try/catch` around `simpMatch?`) that was added in the
first commit.
## Test plan
- [x] Existing test `tests/lean/run/issue12495.lean` passes
- [x] Full test suite (3561 tests) passes with 0 failures
🤖 Generated with [Claude Code](https://claude.com/claude-code)
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Co-authored-by: Claude Opus 4.6 <noreply@anthropic.com>
