Codegen bug: doesn't use qualified type in sqlType invocation

[parsonsmatt]

This showed up in the migration to GHC 9.6 in our codebase. A type that should have been qualified was not. For some reason, GHC accepted this program with 9.4, but 9.6 it complained.

[parsonsmatt]

Huh - relevant code is here:

ftToType :: FieldType -> Type
ftToType = \case
    FTTypeCon Nothing t ->
        ConT $ mkName $ T.unpack t
    -- This type is generated from the Quasi-Quoter.
    -- Adding this special case avoids users needing to import Data.Int
    FTTypeCon (Just "Data.Int") "Int64" ->
        ConT ''Int64
    FTTypeCon (Just m) t ->
        ConT $ mkName $ unpack $ concat [m, ".", t]
    FTLit l ->
        LitT (typeLitToTyLit l)
    FTTypePromoted t ->
        PromotedT $ mkName $ T.unpack t
    FTApp x y ->
        ftToType x `AppT` ftToType y
    FTList x ->
        ListT `AppT` ftToType x

We are doing the qualification, but it's doing ConT (mkName (module . type)). I bet that's the bug - TH is changing how it reads that in.

[parsonsmatt]

Upstream GHC issue. Weirdly enough, it doesn't happen in GHCi - all GHCs work fine 🤔

[alt-romes]

I haven't been able to reproduce the issue, with the minimizer in the ghc's issue tracker, under any version.

However, "this special case avoids users needing to import Data.Int" doesn't sound quite right.
The following module compiles successfully

{-# LANGUAGE TemplateHaskell #-}
module A where

import Data.Int
import Language.Haskell.TH

import Data.Proxy

a :: Proxy $(conT (mkName "Data.Int.Int64"))
a = Proxy

, but if you remove the Data.Int import, we get the following error message, including in 9.4. I believe the user must import the module nonetheless.

blah.hs:8:13: error:
    • Not in scope: type constructor or class ‘Data.Int.Int64’
      NB: no module named ‘Data.Int’ is imported.
    • In the untyped splice: $(conT (mkName "Data.Int.Int64"))
  |
  | a :: Proxy $(conT (mkName "Data.Int.Int64"))

[parsonsmatt]

Yeah, it's a red herring - there's another spot in the code that's doing an unqualified mention.

Which makes me wonder how the 9.4 stuff ever worked 🤔