This PR wraps the top-level command parser with `withPosition` to
enforce indentation in `by` blocks, combined with an empty-by fallback
for better error messages.
This subsumes #3215 (which introduced `withPosition commandParser` but
without the empty-by fallback). It is also related to #9524, which
explores elaboration with empty tactic sequences — this PR reuses that
idea for the empty-by fallback, so that a `by` not followed by an
indented tactic produces an elaboration error (unsolved goals) rather
than a parse error.
**Changes:**
- `topLevelCommandParserFn` now uses `(withPosition commandParser).fn`,
setting the saved position at the start of each top-level command
- `tacticSeqIndentGt` gains an empty tactic sequence fallback
(`pushNone`) so that missing indentation produces an elaboration error
(unsolved goals) instead of a parse error
- `isEmptyBy` in `goalsAt?` removed: with strict `by` indentation, empty
`by` blocks parse successfully via `pushNone` (producing empty nodes)
rather than producing `.missing` syntax, making the `isEmptyBy` check
dead code. The `isEmpty` helper in `isSyntheticTacticCompletion`
continues to work correctly because it handles both `.missing` and empty
nodes from `pushNone` (via the vacuously-true `args.all isEmpty` on
`#[]`)
- Test files updated to indent `by` blocks and expression continuations
that were previously at column 0
**Behavior:**
- Top-level `by` blocks now require indentation (column > 0 for commands
at column 0)
- Commands indented inside `section` require proofs to be indented past
the command's column
- `#guard_msgs in example : True := by` works because tactic indentation
is checked against the outermost command's column
- Expression continuations (not just `by`) must also be indented past
the command, which is slightly more strict but more consistent
- `have : True := by` followed by a dedent now correctly puts `this` in
scope in the outer tactic block (the `have` is structurally complete
with an unsolved-goal error, rather than a parse error)
**Code changes observed in practice (lean4 test suite + Mathlib):**
- `by` blocks: top-level `theorem ... := by` / `decreasing_by` followed
by tactics at column 0 must be indented
- `variable` continuations: `variable {A : Type*} [Foo A]\n{B : Type*}`
where the second line starts at column 0 must be indented (most common
category in Mathlib)
- Expression continuations: `def f : T :=\nexpr` or `#synth Foo\n[args]`
where the body/arguments start at column 0
- Structure literals: `.symm\n{ toFun := ...` where the struct literal
starts at column 0
🤖 Generated with [Claude Code](https://claude.com/claude-code)
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>
---------
Co-authored-by: Claude Opus 4.6 <noreply@anthropic.com>
