Verso docstrings on `where`-clause declaration cause errors

[Vierkantor]

Prerequisites

• Check that your issue is not already filed:
  https://github.com/leanprover/lean4/issues
• Reduce the issue to a minimal, self-contained, reproducible test case.
  Avoid dependencies to Mathlib or Batteries.
• 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

Plain text docstrings are allowed on declarations in a where clause, but Verso docstrings cause an error: "Unknown constant _ when building docstring". For example:

set_option doc.verso true

/-- This docstring works. -/
def foo := bar
  where
  /-- This docstring is not allowed:
  -- error: Unknown constant foo.bar when building docstring
   -/
  bar := 37

-- But with Verso docs disabled:
set_option doc.verso false

/-- This docstring works. -/
def foo' := bar'
  where
  /-- This docstring now also works. -/
  bar' := 37

Context

Mathlib uses docstrings, rather than plain comments, to document the declarations in a where clause.

Steps to Reproduce

1. Enable the option doc.verso.
2. Write a def with a where clause.
3. Add a declaration to that where clause.
4. Add a docstring to that declaration.

Expected behavior: The docstring does not throw any errors.

Actual behavior: There is an error "Unknown constant _ when building docstring".

Versions

Lean 4.27.0-rc1 (Mathlib, MacOS / Darwin Kernel Version 23.6.0 / arm64), Lean 4.28.0-nightly-2026-01-19 (Lean4web nightly)

Additional Information

n/a

Impact

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[alok]

TIL where blocks can have docs