TH-spliced pattern synonym declaration fails to typecheck

This pattern synonyms typechecks without issue:

{-# LANGUAGE GADTs #-}
{-# LANGUAGE PatternSynonyms #-}
module Works where

data T a where
  MkT :: Eq b => b -> T a

pattern P :: b -> T a
pattern P x <- MkT x

But if you try to create P from a Template Haskell splice, it will fail to typecheck:

{-# LANGUAGE GADTs #-}
{-# LANGUAGE PatternSynonyms #-}
{-# LANGUAGE TemplateHaskell #-}
module Bug where

data T a where
  MkT :: Eq b => b -> T a

$([d| pattern P :: b -> T a; pattern P x <- MkT x |])

$ /opt/ghc/head/bin/ghc Bug.hs
[1 of 1] Compiling Bug              ( Bug.hs, Bug.o )

Bug.hs:9:3: error:
    • Couldn't match expected type ‘b0’ with actual type ‘b’
      ‘b’ is a rigid type variable bound by
        a pattern with constructor: MkT :: forall a b. Eq b => b -> T a,
        in a pattern synonym declaration
        at Bug.hs:9:3-52
      ‘b0’ is a rigid type variable bound by
        the signature for pattern synonym ‘P’ at Bug.hs:9:3-52
    • In the declaration for pattern synonym ‘P’
    • Relevant bindings include x_a4tP :: b (bound at Bug.hs:9:3)

Trac metadata

Trac field	Value
Version	8.1
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Template Haskell
Test case
Differential revisions
BlockedBy
Related
Blocking
CC	PatternSynonyms
Operating system
Architecture