Pattern synonym signatures are not validity-checked

Ponder this garbage:

{-# LANGUAGE PatternSynonyms, ViewPatterns,
             TypeFamilies, KindSignatures #-}

module Bug where

type family F a  -- F :: * -> *
data T :: (* -> *) -> *

pattern Q :: T F -> String
pattern Q x <- (undefined -> x)

This is accepted by GHC 8.0.1 and HEAD. But it has an unsaturated type family! Urk!

The problem is that TcSigs.tcPatSynSig never does validity checking.

Trac metadata

Trac field	Value
Version	8.1
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler (Type checker)
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