Document GHC-61689; Add examples to GHC-88464 and GHC-76037

[tbidne]

Hello!

I recently ran into the error like

E2NoFieldSelectors.hs:11:24: error: [GHC-61689]
    Module ‘A’ does not export ‘fint’.
    Suggested fix:
      Notice that ‘fint’ is a field selector belonging to the type ‘A.Foo’
      that has been suppressed by NoFieldSelectors.
   |
11 | import A (Foo (MkFoo), fint)
   |                        ^^^^

And I found the various nuances confusing enough that I thought it merited a description here. It turns out there are multiple errors that can trigger this message.

GHC-61689 (New)

This is a new error introduced in GHC 9.8.1 for importing a an unexported symbol.

-- GHC-61689, fint is a field selector disabled with NoFieldSelectors, always an error
import A (Foo (MkFoo), fint)

GHC-88464

This was previously documented in other cases, but it can also trigger the NoFieldSelectors message in the right circumstances.

-- GHC-88464, error if fint is used as a selector, OK otherwise
import A (Foo (MkFoo, fint))

getFooInt :: Foo -> Int
getFooInt = fint -- this is the problem

GHC-76037

Similar to GHC-61689, though it involves the export, not the import.

{-# LANGUAGE NoFieldSelectors #-}

-- GHC-76037, error if foo is exported at the top-level rather than part of T.
module NoFieldSelectorsExport (T, foo) where

data T = MkT { foo :: T }

I have this in the same PR as they are all related, but please let me know if they should be split up, or if there should be issues created. Thanks!

[jhrcek]

The examples and explanations look good. Few minor suggestions.

[jhrcek]

Thanks! Looks good to me.

[tbidne]

Thanks for the review!