Improve error recovery in the typechecker
Issue Trac #16418 showed that we were carrying on too eagerly after a bogus
type signature was identified (a bad telescope in fact), leading to a
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.
- 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.
(cherry picked from commit 4927117cd6166a97455d788dbf7433c95441b57a)