Commit 01c948eb authored by Committed by Marge Bot
Clean up the inferred type variable restriction
This patch primarily: * Documents `checkInferredVars` (previously called `check_inferred_vars`) more carefully. This is the function which throws an error message if a user quantifies an inferred type variable in a place where specificity cannot be observed. See `Note [Unobservably inferred type variables]` in `GHC.Rename.HsType`. Note that I now invoke `checkInferredVars` _alongside_ `rnHsSigType`, `rnHsWcSigType`, etc. rather than doing so _inside_ of these functions. This results in slightly more call sites for `checkInferredVars`, but it makes it much easier to enumerate the spots where the inferred type variable restriction comes into effect. * Removes the inferred type variable restriction for default method type signatures, per the discussion in #18432. As a result, this patch fixes #18432. Along the way, I performed some various cleanup: * I moved `no_nested_foralls_contexts_err` into `GHC.Rename.Utils` (under the new name `noNestedForallsContextsErr`), since it now needs to be invoked from multiple modules. I also added a helper function `addNoNestedForallsContextsErr` that throws the error message after producing it, as this is a common idiom. * In order to ensure that users cannot sneak inferred type variables into `SPECIALISE instance` pragmas by way of nested `forall`s, I now invoke `addNoNestedForallsContextsErr` when renaming `SPECIALISE instance` pragmas, much like when we rename normal instance declarations. (This probably should have originally been done as a part of the fix for #18240, but this task was somehow overlooked.) As a result, this patch fixes #18455 as a side effect.