View Patterns affect typechecking in an unpredictable manner

Copying the text from my StackOverflow question.

{-# LANGUAGE ViewPatterns #-}

import Data.Text (Text)
import qualified Data.Text as T
import qualified Data.Vector.Unboxed as UV
import qualified Data.Vector.Generic as V

bar :: Int -> UV.Vector Char -> (Text, Text)
bar i v = (t_pre, t_post)
  where
    f = T.pack . V.toList
    (f -> t_pre, f -> t_post) = V.splitAt i v

This gives an unexpected type error (tested with GHC 8.4.3), concerning ambiguous type variables and f being out of scope.

    • Ambiguous type variable ‘v0’ arising from a use of ‘V.toList’
      prevents the constraint ‘(V.Vector v0 Char)’ from being solved.
      Relevant bindings include
    f :: v0 Char -> Text (bound at Weird.hs:11:5)
      Probable fix: use a type annotation to specify what ‘v0’ should be.
      These potential instances exist:
    instance V.Vector UV.Vector Char
      -- Defined in ‘Data.Vector.Unboxed.Base’
    ...plus one instance involving out-of-scope types
      instance primitive-0.6.3.0:Data.Primitive.Types.Prim a =>
               V.Vector Data.Vector.Primitive.Vector a
        -- Defined in ‘Data.Vector.Primitive’
    • In the second argument of ‘(.)’, namely ‘V.toList’
      In the expression: T.pack . V.toList
      In an equation for ‘f’: f = T.pack . V.toList
   |
11 |     f = T.pack . V.toList
   |                  ^^^^^^^^

Weird.hs:13:6: error:
    Variable not in scope: f :: UV.Vector Char -> t
   |
13 |     (f -> t_pre, f -> t_post) = V.splitAt i v
   |      ^

Weird.hs:13:18: error:
    Variable not in scope: f :: UV.Vector Char -> t1
   |
13 |     (f -> t_pre, f -> t_post) = V.splitAt i v
   |                  ^

Some of the other answers on SO have done some digging (e.g. enabling FlexibleContexts / NoMonomorphismRestriction doesn't solve the issue). I haven't yet figured out how to minimize the example further to remove the dependency on Vector...

Is this a compiler bug or a documentation bug?

Trac metadata

Trac field	Value
Version	8.4.3
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler
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