Commit 4927117c authored by Committed by Marge Bot
Improve error recovery in the typechecker
Issue #16418 showed that we were carrying on too eagerly after a bogus type signature was identified (a bad telescope in fact), leading to a subsequent crash. This led me in to a maze of twisty little passages in the typechecker's error recovery, and I ended up doing some refactoring in TcRnMonad. Some specfifics * TcRnMonad.try_m is now called attemptM. * I switched the order of the result pair in tryTc, to make it consistent with other similar functions. * The actual exception used in the Tc monad is irrelevant so, to avoid polluting type signatures, I made tcTryM, a simple wrapper around tryM, and used it. The more important changes are in * TcSimplify.captureTopConstraints, where we should have been calling simplifyTop rather than reportUnsolved, so that levity defaulting takes place properly. * TcUnify.emitResidualTvConstraint, where we need to set the correct status for a new implication constraint. (Previously we ended up with an Insoluble constraint wrapped in an Unsolved implication, which meant that insolubleWC gave the wrong answer.
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