GHCi accepts invalid programs when recompiling

Here's what's happening:

Module A imports module B. Module B contains a function with constraints that are required to compile module B. If I load module A in GHCi, I can remove some (required) constraints on the function in module B and GHCi will successfully reload module B.

My example uses the syntactic library. I attempted to recreate the situation I just described without syntactic, but I was unsuccessful.

Module A.hs:

module A where

import B
import Data.Syntactic.Sugar.BindingT ()

main = print "hello"

Module B.hs:

{-# LANGUAGE GADTs, TypeOperators, FlexibleContexts #-}

module B where

import Data.Syntactic

data Let x where
  Let :: Let (a :-> (a -> b) :-> Full b)

share :: (Let :<: sup,
          sup ~ Domain b, sup ~ Domain a,
          Internal (a -> b) ~ (Internal a -> Internal b), -- remove me
          Syntactic a, Syntactic b,
          Syntactic (a -> b),
          SyntacticN (a -> (a -> b) -> b) 
            (ASTF sup (Internal a) ->
              ASTF sup (Internal (a -> b)) ->
               ASTF sup (Internal b)))
      => a -> (a -> b) -> b
share = sugarSym Let

Here's a terminal transcript:

$ ghci A
[1 of 2] Compiling B    ( testsuite/B.hs, interpreted )
[2 of 2] Compiling A    ( testsuite/A.hs, interpreted )
Ok, modules loaded: A, B.
>
(Now remove the constraint from B and save. This *should* break module B)
> :r
[1 of 2] Compiling B    ( testsuite/B.hs, interpreted )
Ok, modules loaded: A, B.
> :q
$ ghci B
[1 of 2] Compiling B    ( testsuite/B.hs, interpreted )

testsuite/B.hs:21:9:
    Could not deduce (Internal (a -> b) ~ (Internal a -> Internal b))

If I had to guess what's going on, it's that GHCi is remembering the instance that A imports from BindingT:

instance (...) => Syntactic (a -> b) where
  type Internal (a -> b) = Internal a -> Internal b

This instance implies the constraint that module B needs, however it should never be visible to module B. GHCi seems to be ignoring that and using the instance to recompile module B.

When compiling from scratch, module B is compiled first, so of course the instance (and therefore the constraint) are not visible.

Trac metadata

Trac field	Value
Version	7.8.3
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	GHCi
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CC	hvr
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