unexpected error when elaborating 'let'

[Rob23oba]

Prerequisites

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• Reduce the issue to a minimal, self-contained, reproducible test case.
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Description

For some combinations of if statements, for loops, let declarations and mutation inside of a do block, the error unexpected error while elaborating 'let' is produced.

Example

example (arr : Array Nat) : Unit := Id.run do
  let mut abc := 0
  if 0 = 0 then
    ()
  for (i, j) in [(0, 0)] do -- unexpected error when elaborating 'let'
    let a : Nat := i + 2
    if h : arr.size ≤ 4 then
      continue
    else if h : arr[4] ≤ a then
      continue
    else
      if 0 = 0 then
        continue
      abc := 0

If you change the last part to

let b := arr[0]
if b = 0 then
  continue
abc := ()

then the error goes away.

Versions

Lean 4.16.0-nightly-2024-12-20 (on live.lean-lang.org)
Lean 4.14.0-rc2

Impact

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