Inlining leads to "failed to compile definition"

[MiddleAdjunction]

Prerequisites

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Description

Consider the following simple definition of data C : Type and binary relation Eqv : C → C → Type over it.

inductive C : Type
| C0 : C
| C1 : C → C
open C

-- @[inline] -- <-- [HERE]
def id_C (c : C) : C :=
  match c with
  | C0    => C0
  | C1 c' => C1 (id_C c')

inductive Eqv : C → C → Type where
| Refl : Eqv c        c
| IdC  : Eqv (id_C c) c
open Eqv

def id_Eqv  (eq : Eqv c₁ c₂) : Eqv c₁ c₂ :=
  match eq with
  | Refl => Refl
  | IdC  => IdC

Notice that the relation introduces an equality on indices that involves a simple recursive function id_C.

Adding the @[inline] annotation for id_C results in the following error message:

failed to compile definition, consider marking it as 'noncomputable' because it depends on 'Original.id_Eqv.match_1', and it does not have executable code

This is not necessarily a bug in Lean's internals if inline is not meant to be used on recursive functions. In that case, there should arguably be a clear message warning about the misuse of inline, as the current error can be quite misleading.

Context

I raised this issue on a Discord server (Lean 4 anarchy); the description above summarises the issue fairly well.

Steps to Reproduce

Run the code above with @[inline] annotation on id_C (i.e., uncomment the part labelled [HERE]).

Expected behavior: There should arguably be at least a clear message warning about the misuse of inline on id_C.

Actual behavior: User gets the following cryptic error message pointing to id_Eqv:

failed to compile definition, consider marking it as 'noncomputable' because it depends on 'Original.id_Eqv.match_1', and it does not have executable code

Versions

[4.12.0-nightly-2024-10-22]

Impact

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