Failure of exhaustiveness checker when matching on `Exception`

JLimperg commented 6 days ago

Prerequisites

Please put an X between the brackets as you perform the following steps:

[X] Check that your issue is not already filed: https://github.com/leanprover/lean4/issues
[X] Reduce the issue to a minimal, self-contained, reproducible test case. Avoid dependencies to Mathlib or Batteries.
[X] Test your test case against the latest nightly release, for example on https://live.lean-lang.org/#project=lean-nightly (You can also use the settings there to switch to “Lean nightly”)

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 commented 5 days ago

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 commented 5 days ago

Workaround.

import Lean

open Lean

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

JLimperg commented 5 days ago

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.

leanprover / lean4