Before, the termination argument as inferred by `GuessLex` was passed
further
on as `Syntax`, to be elaborated later in `WF.Rel`.
This didn’t feel quite right anymore. In particular if we want to teach
`GuessLex` about guessing more complex termination arguments like
`xs.size -
i`, using `Expr` here is more natural.
So this introduces `TerminationArgument` based on an `Expr` to be used
here.
A side-effect of how the termination arguments are elaborated is that
the unused
variables linter will now look at `termination_by` variables, and that
parameters
past the colon are not even invisibly in scope, so `‹_›` will not find
them
See https://github.com/leanprover-community/mathlib4/pull/11370/files
for examples
of fixing these changes.
Floris van Doorn [reported on
Zulip](https://leanprover.zulipchat.com/#narrow/stream/270676-lean4/topic/have.20tactic.20error.20recovery/near/425283053)
that it is confusing that the `have : T := e` tactic completely fails if
the body `e` is not of type `T`. This is in contrast to `have : T := by
exact e`, which does not completely fail when `e` is not of type `T`.
This ends up being caused by `elabTermEnsuringType` throwing an error
when it fails to insert a coercion. Now, it detects this case, and it
checks the `errToSorry` flag to decide whether to throw the error or to
log the error and insert a `sorry`.
This is justified by `elabTermEnsuringType` being a frontend to
`elabTerm`, which inserts `sorry` on error.
An alternative would be to make `ensureType` respect `errToSorry`, but
there exists code that expects being able to catch when `ensureType`
fails. Making such code manipulate `errToSorry` seems error prone, and
this function is not a main entry point to the term elaborator, unlike
`elabTermEnsuringType`.
