Can't derive SizeOf for public inductive with private constructor

[david-christiansen]

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Description

In nightlies after 2026-04-08, I get a type error in the derived SizeOf instance for an inductive type with a public and a private constructor. MWE:

module

public inductive Tag where
  | provided
  | private external

Error:

failed to generate `SizeOf` instance for `Tag`:
  type mismatch

Context

This is minimized from an example in Verso, where the private constructor is used to ensure that internal code has made a provided name unique.

Steps to Reproduce

1. Use the latest nightly
2. Enter the code
3. Observe the error

Expected behavior:
No error message.

Actual behavior:

The described error:

failed to generate `SizeOf` instance for `Tag`:
  type mismatch

Versions

Lean 4.31.0-nightly-2026-04-11
Target: x86_64-unknown-linux-gnu

Additional Information

Impact

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[nomeata]

Thanks, I (well Claude) is on it, a simple missing withoutExporting it seems.

[david-christiansen]

Thank you!