Partial type signatures in class constraints behave unexpectedly

Minimal example:

{-# LANGUAGE PartialTypeSignatures #-}
{-# LANGUAGE TemplateHaskell #-}
module Minimal where

id [d|
    f :: (Monad m, _) => [m a] -> m [a]
    f' :: (Monad m, _) => [m a] -> m [a]
    f = f'
    f' [] = return []
    f' (x:xx) = f xx
  |]

[1 of 1] Compiling Minimal          ( /Users/pepe/Dropbox/code/debug-hoed/test/minimal.hs, interpreted )

/Users/pepe/Dropbox/code/debug-hoed/test/minimal.hs:5:1: warning: [-Wpartial-type-signatures]
    • Found type wildcard ‘_’ standing for ‘()’
    • In the type signature:
        f :: (Monad m_a7NN, _) => [m_a7NN a_a7NO] -> m_a7NN [a_a7NO]
  |
5 | id [d|
  | ^^^^^^...

/Users/pepe/Dropbox/code/debug-hoed/test/minimal.hs:5:1: warning: [-Wpartial-type-signatures]
    • Found type wildcard ‘_’ standing for ‘()’
    • In the type signature:
        f' :: (Monad m_a7NL, _) => [m_a7NL a_a7NM] -> m_a7NL [a_a7NM]
  |
5 | id [d|
  | ^^^^^^...
Ok, one module loaded.
 :browse
f :: (Monad m, Monad m) => [m a] -> m [a]
f' :: (Monad m, Monad m) => [m a] -> m [a]

Notice the duplicate Monad m constraint.

Things get even more weird if the type signatures are declared together:

id [d|
    f, f' :: (Monad m, _) => [m a] -> m [a]
    f = f'
    f' [] = return []
    f' (x:xx) = f xx
     |]

[1 of 1] Compiling Minimal          ( /Users/pepe/Dropbox/code/debug-hoed/test/minimal.hs, interpreted )

/Users/pepe/Dropbox/code/debug-hoed/test/minimal.hs:5:1: warning: [-Wpartial-type-signatures]
    • Found type wildcard ‘_’ standing for ‘()’
    • In the type signature:
        f :: (Monad m_a88E, _) => [m_a88E a_a88F] -> m_a88E [a_a88F]
  |
5 | id [d|
  | ^^^^^^...

/Users/pepe/Dropbox/code/debug-hoed/test/minimal.hs:5:1: warning: [-Wpartial-type-signatures]
    • Found type wildcard ‘_’ standing for ‘()’
    • In the type signature:
        f' :: (Monad m_a88E, _) => [m_a88E a_a88F] -> m_a88E [a_a88F]
  |
5 | id [d|
  | ^^^^^^...
Ok, one module loaded.
 :browse
f ::
  (Monad ghc-prim-0.5.1.1:GHC.Types.Any, Monad m) =>
  [ghc-prim-0.5.1.1:GHC.Types.Any ghc-prim-0.5.1.1:GHC.Types.Any]
  -> ghc-prim-0.5.1.1:GHC.Types.Any [ghc-prim-0.5.1.1:GHC.Types.Any]
f' ::
  (Monad ghc-prim-0.5.1.1:GHC.Types.Any, Monad m) => [m a] -> m [a]

Trac metadata

Trac field	Value
Version	8.2.2
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler
Test case
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