Underapplied pattern synonyms expand to lambdas with wrong hiding in expressions

[Andreas Abel]

data D : Set where
  c : D → {y : D} → D → D

pattern p x {y} z = c x {y} z

f : D → {y : D} → D → D
f = p

Error:

(z : D) → D !=< D
when checking that the expression λ z → p x {y} z has type D

This is already broken in 2.4.2.4 (the oldest Agda I have at my disposal).

Found this bug when reading the code:

agda/src/full/Agda/TypeChecking/Rules/Application.hs

Lines 144 to 156 in 7ad9543

 A.PatternSyn n -> do 
   (ns, p) <- lookupPatternSyn n 
   p <- return $ setRange (getRange n) $ killRange $ vacuous p   -- Pattern' Void -> Pattern' Expr 
   -- Expand the pattern synonym by substituting for 
   -- the arguments we have got and lambda-lifting 
   -- over the ones we haven't. 
   let meta r = A.Underscore $ A.emptyMetaInfo{ A.metaRange = r }   -- TODO: name suggestion 
   case A.insertImplicitPatSynArgs meta (getRange n) ns args of 
     Nothing      -> typeError $ BadArgumentsToPatternSynonym n 
     Just (s, ns) -> do 
       let p' = A.patternToExpr p 
           e' = A.lambdaLiftExpr (map whThing ns) (A.substExpr s p') 
       checkExpr' cmp e' t 

lambdaLiftExpr does not take the hiding into account (it is discarded), so the generated lambdas have the wrong hiding.