Template Haskell + hs-boot = Not in scope during type checking, but it passed the renamer

I don't think this is a very harmful bug but it definitely is a bug. Consider:

-- A.hs-boot
module A where
data T

-- B.hs
module B (module A) where
import {-# SOURCE #-} A

-- A.hs
{-# LANGUAGE TemplateHaskell #-}
module A where
import qualified B
import Language.Haskell.TH
f :: B.T -> B.T
f x = x
$( return [] )
data T = T B.T

The point of the splice is to convince GHC to typecheck f :: B.T -> B.T before it typechecks the type declaration. But this is not going to work, because tcLookupGlobal is going to bail if (1) T is not in the tcg_type_env and (2) T comes from this module.

It would be a simple matter to improve the error message but from a user's perspective, there is no good reason for this to not typecheck. On the implementation side, I am sympathetic to not letting this typecheck: if it does typecheck, then some TyCons will incorrectly refer to the definition from the hs-boot file, rather than our local definition.

Trac metadata

Trac field	Value
Version	8.0.1-rc2
Type	Bug
TypeOfFailure	OtherFailure
Priority	low
Resolution	Unresolved
Component	Compiler (Type checker)
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