Type synonym with context in pattern synonym

pattern :: (Num a, Eq a) => a
pattern Fiskur = 42

unlike

type Diskur a = Num a => a

fiskur :: Num a => a
fiskur = 42

we cannot write

type Fugl a = (Num a, Eq a) => a

pattern Fiskur :: Fugl a
pattern Fiskur = 42

-- Exp.hs:54:18-19: error: …
--     • No instance for (Eq a) arising from a pattern
--       Possible fix:
--         add (Eq a) to the context of
--           the type signature for pattern synonym ‘Fiskur’:
--             Fugl a
--     • In the pattern: 42
--       In the declaration for pattern synonym ‘Fiskur’
-- Compilation failed.

Given that the story of pattern synonym contexts is more complicated than usual patterns given provided constraints, I know there are other issues (#11524 (closed), #10928 (closed)).

---

For fun why not type synonym with provided and required constraints:

data T a where
  MkT :: (Show b) => a -> b -> T a

f1 :: (Eq a, Num a) => T a -> String
f1 (MkT 42 x) = show x

type MKNUMPAT a b = (Num a, Eq a) => (Show b) => b -> T a

-- pattern ExNumPat :: (Num a, Eq a) => (Show b) => b -> T a
pattern ExNumPat :: MKNUMPAT a b
pattern ExNumPat x = MkT 42 x

What would that mean in a regular type signature? CE := CP + CR where EXNUMPAT a b means the same as EXNUMPAT' a b? (#8581)

type MKNUMPAT' a b = (Num a, Eq a, Show b) => b -> T a

-- This works:
mkNumPat :: MKNUMPAT a b
mkNumPat = ExNumPat

Trac metadata

Trac field	Value
Version	8.1
Type	FeatureRequest
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler
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