Failure of exhaustiveness checker when matching on Exception

[JLimperg]

Prerequisites

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Description

MWE:

import Lean

open Lean

example (e : Exception) : True :=
  match e with
  | .internal .. => sorry
  | .error .. => sorry

-- missing cases:
-- (@Exception.internal _ (KVMap.mk (List.cons (Prod.mk _ (DataValue.ofSyntax _)) _)))
-- (@Exception.internal _ (KVMap.mk (List.cons (Prod.mk _ (DataValue.ofInt (Int.negSucc _))) _)))
-- (@Exception.internal _ (KVMap.mk (List.cons (Prod.mk _ (DataValue.ofInt (Int.ofNat _))) _)))
-- (@Exception.internal _ (KVMap.mk (List.cons (Prod.mk _ (DataValue.ofNat _)) _)))
-- (@Exception.internal _ (KVMap.mk (List.cons (Prod.mk _ (DataValue.ofName _)) _)))
-- (@Exception.internal _ (KVMap.mk (List.cons (Prod.mk _ (DataValue.ofBool true)) _)))
-- (@Exception.internal _ (KVMap.mk (List.cons (Prod.mk _ (DataValue.ofBool false)) _)))
-- (@Exception.internal _ (KVMap.mk (List.cons (Prod.mk _ (DataValue.ofString (String.mk _))) _)))

Versions

"4.10.0-nightly-2024-06-24"

Impact

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[leodemoura]

The issue here is that Exception.internal has a default argument.

inductive Exception where
  | error (ref : Syntax) (msg : MessageData)
  | internal (id : InternalExceptionId) (extra : KVMap := {})

Thus, .internal .. is equivalent to .internal _ {}. Possible solutions:

• A better error message.
• Modify the meaning of ...

[leodemoura]

Workaround.

import Lean

open Lean

example (e : Exception) : True :=
  match e with
  | .internal _ _ => sorry
  | .error .. => sorry

[JLimperg]

Ah, it's the default argument! I understand .. as "ignore all other arguments", so I would prefer if there was no distinction between arguments with and without a default value.