Instanceness is lost when expanding absurd pattern in pattern synonym expression

[Andreas Abel]

open import Agda.Builtin.Bool
open import Agda.Builtin.Equality

data D : Set where
  c : {{ i : true ≡ false }} → D

pattern ff = c {{ () }}

works : D → D
works ff

works' : D → D
works' c = c

test : D → D
test c = ff

The test fails with an unsolved constraint:

_i_12
  : Agda.Builtin.Bool.Bool.true Agda.Builtin.Equality.≡
    Agda.Builtin.Bool.Bool.false

The code to translate a pattern synonym with absurd pattern to an expression is this one (last line):

agda/src/full/Agda/Syntax/Abstract.hs

Lines 997 to 1006 in fc866f9

 patternToExpr :: Pattern -> Expr 
 patternToExpr = \case 
   VarP x             -> Var (unBind x) 
   ConP _ c ps        -> Con c `app` map (fmap (fmap patternToExpr)) ps 
   ProjP _ o ds       -> Proj o ds 
   DefP _ fs ps       -> Def (headAmbQ fs) `app` map (fmap (fmap patternToExpr)) ps 
   WildP _            -> Underscore emptyMetaInfo 
   AsP _ _ p          -> patternToExpr p 
   DotP _ e           -> e 
   AbsurdP _          -> Underscore emptyMetaInfo  -- TODO: could this happen? 

There are two problems:

1. The meta info has no scope info, thus the constraint is printed in the top-level scope (full qualification).
   • #7177
2. The absurd pattern is in instance position and should turn into an instance meta, but it does not.

The second problem is a consequence of rejecting the semantics of {{_}} suggested in #2172, but after #7173 we have some machinery to fix this.