luafmt is not idempotent: layout of a nested call alternates between two states forever #1302

This input never stabilizes: from the 3rd pass on, luafmt flips between two layouts forever (pass 3 == pass 5, pass 3 != pass 4), so luafmt --check fails on its own output.

Save as osc.lua:

function M.evidence(rows)
  local facts = Seq.from(rows)
    :map(function(row)
      local p = row[2]
      return Seq.concat(
        { p.tail ~= '' and p.tail or '[no name]', 'ft=' .. (p.filetype ~= '' and p.filetype or '-') },
        {
          'lines=' .. p.lines,
        },
        p.marks ~= nil and { 'marks=' .. p.marks } or {},
        p.modified and { 'MODIFIED' } or {}
      ):join '  '
    end)
    :totable()
end

cp osc.lua o0.lua
for i in 1 2 3 4 5; do luafmt --max-line-width 120 < o$((i-1)).lua > o$i.lua; done
cmp o3.lua o5.lua && ! cmp -s o3.lua o4.lua && echo '2-cycle'

The two states alternate between the argument list being "packed" on the Seq.concat( line and fully expanded one-per-line:

      return Seq.concat({ p.tail ~= '' and p.tail or '[no name]', 'ft=' .. (p.filetype ~= '' and p.filetype or '-') }, {
        'lines=' .. p.lines,
      }, p.marks ~= nil and { 'marks=' .. p.marks } or {}, p.modified and { 'MODIFIED' } or {}
      ):join '  '

Without the enclosing function ... end (two columns less indentation) it converges after one pass. The first pass also emitted a 123-column line elsewhere in the same codebase with max_line_width = 120, which the second pass then split.

On a 165-file codebase, 25 files changed again on the second pass, 5 on the third, and 2 kept alternating. --verify reports this, but there's no way to reach a fixed point.

luafmt 0.25.1.