`PANIC at _private.Lean.Environment.0.Lean.AsyncConsts.add`

Prerequisites

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Description

Here is some invalid code:

structure A where
  []

It leads to the following slightly scary error message (or is it even an error message? It's actually an info diagnostic I think):

PANIC at _private.Lean.Environment.0.Lean.AsyncConsts.add Lean.Environment:432:4: duplicate normalized declaration name A vs. A
backtrace:
PANIC at _private.Lean.Environment.0.Lean.AsyncConsts.add Lean.Environment:432:4: duplicate normalized declaration name A vs. A
backtrace:

Expected behavior: There is already an error message unexpected token ']'; expected identifier which is probably enough.

Actual behavior: An additional scary panic message is printed twice.

Versions

4.21.0-nightly-2025-06-03 on live.lean-lang.org

Impact

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Reproduce the invalid structure A where [] snippet on the Lean nightly release or live.lean-lang.org and inspect the duplicate diagnostic path reported at Lean.Environment:432:4. Done means the existing unexpected token ']'; expected identifier diagnostic remains, without the additional panic being printed twice.