fix: make elabTermEnsuringType respect errToSorry when there is a type mismatch
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leodemoura
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… type mismatch 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` fails to have type `T`. This ends up being caused by `elabTermEnsuringType` throwing an error when it fails inserting 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 catch errors from this function.
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Floris van Doorn reported on Zulip that it is confusing that the
have : T := etactic completely fails if the bodyeis not of typeT. This is in contrast tohave : T := by exact e, which does not completely fail wheneis not of typeT.This ends up being caused by
elabTermEnsuringTypethrowing an error when it fails to insert a coercion. Now, it detects this case, and it checks theerrToSorryflag to decide whether to throw the error or to log the error and insert asorry.This is justified by
elabTermEnsuringTypebeing a frontend toelabTerm, which insertssorryon error.An alternative would be to make
ensureTyperespecterrToSorry, but there exists code that expects being able to catch whenensureTypefails. Making such code manipulateerrToSorryseems error prone, and this function is not a main entry point to the term elaborator, unlikeelabTermEnsuringType.