sheaf authored Jan 04, 2022 and Marge Bot committed Jan 11, 2022

The `GHC.Tc.Plugin.newWanted` function takes a `CtLoc` as an argument,
but it used to discard the location information, keeping only
the `CtOrigin`. It would then retrieve the source location from the
`TcM` environment using `getCtLocM`.

This patch changes this so that `GHC.Tc.Plugin.newWanted` passes on
the full `CtLoc`. This means that authors of type-checking plugins
no longer need to manually set the `CtLoc` environment in the `TcM`
monad if they want to create a new Wanted constraint with the given
`CtLoc` (in particular, for setting the `SrcSpan` of an emitted
constraint). This makes the `newWanted` function consistent with
`newGiven`, which always used the full `CtLoc` instead of using
the environment.

Fixes #20895