Issues

• Sym.dsimp is slower that Sym.simp on heavily nested expressions

  bug
  leanprover/lean4#15938 · Filter by author robin-carlier opened on Oct 25, 2026

• bug: isExclusiveUnsafe should be marked @[never_extract]

  bug
  leanprover/lean4#15925 · Filter by author fgdorais opened on Oct 24, 2026

• #guard segfaults when its term contains sorry

  bug
  leanprover/lean4#15919 · Filter by author 0xAX opened on Oct 24, 2026

• leantar panics (src/lgz.rs:1317, assertion failed: s2.len() == s.len()) archiving a specific .olean

  leanprover/lean4#15907 · Filter by author kevaundray opened on Oct 24, 2026

• RFC: pretty-printing option that disables quantifier chaining

  RFC
  leanprover/lean4#15895 · Filter by author madvorak opened on Oct 23, 2026

• Outside the module system, two modules that realize f.fun_cases_unfolding still cannot both be imported: duplicate _sparseCasesOn auxiliary

  leanprover/lean4#15888 · Filter by author ericeallen opened on Oct 22, 2026
  1

• Std.Http: Header.Name.ofString? and Header.Value.ofString? cost 2.4-3.4× a single pass over the same checks

  bug
  leanprover/lean4#15887 · Filter by author paulbutcher opened on Oct 22, 2026

• Decidable instances with let binders potentially loose sharing during code generation.

  bugcode-generator
  leanprover/lean4#15886 · Filter by author paulbutcher opened on Oct 22, 2026
  2

• def contracts (#14825): given/requires/ensures refused as binder names inside brackets

  leanprover/lean4#15881 · Filter by author kmrasmussen opened on Oct 22, 2026

• Lake cache stores olean under stale source hash if file changes mid-build

  leanprover/lean4#15880 · Filter by author nomeata opened on Oct 22, 2026

• grind?: kernel error with set_option Elab.async false

  bug
  leanprover/lean4#15866 · Filter by author Leo0506 opened on Oct 22, 2026

• RFC: bulk unpack downloaded module archives to reduce cache restoration overhead

  leanprover/lean4#15865 · Filter by author kim-em opened on Oct 22, 2026

• where finally: kernel error when the proof of a hole in a let rec relies on a let value

  bug
  leanprover/lean4#15864 · Filter by author Leo0506 opened on Oct 22, 2026

• [fuzz-tested] LEAN ASSERTION VIOLATION File: src/library/instantiate_mvars.cpp Line: 801 !get_mvar_assignment(m_mctx, mvar_name(e))

  bug
  leanprover/lean4#15849 · Filter by author YuanchengJiang opened on Oct 21, 2026

• lake cache get resuses path for revisions mapping (regardless of scope)

  bug
  leanprover/lean4#15848 · Filter by author arajasek opened on Oct 21, 2026

• Std.Http: valid chunked request gets 400 when a read boundary falls inside a chunk extension or trailer

  leanprover/lean4#15840 · Filter by author eumemic opened on Oct 21, 2026

• RFC: lemmas relating ByteArray.foldlM to Array.foldlM

  leanprover/lean4#15834 · Filter by author ChristianMurphy opened on Oct 21, 2026

• Docstrings for well-founded recusion with multiple arguments can't refer to themselves

  bug
  leanprover/lean4#15828 · Filter by author robsimmons opened on Oct 20, 2026
  1

  

• Lake: package dependency cycles load and build libraries, but every executable fails with "build cycle detected" — are package cycles supported?

  leanprover/lean4#15820 · Filter by author kmrasmussen opened on Oct 20, 2026

• Lake: dynamically constructed build cycle is not detected and hangs

  bug
  leanprover/lean4#15810 · Filter by author dranov opened on Oct 20, 2026

• Lake: concurrent fetch from running jobs builds the same target more than once

  bugP-medium
  leanprover/lean4#15809 · Filter by author dranov opened on Oct 20, 2026
  1

• Lake: collectArray, mixArray, collectVector, collectList, and mixList are quadratic

  bugP-medium
  leanprover/lean4#15808 · Filter by author dranov opened on Oct 20, 2026

• Windows: intermittent stack-cookie failure (0xC0000409) in uv__tcp_connect with Std.Async.TCP connect

  P-high
  leanprover/lean4#15807 · Filter by author gijzelaerr opened on Oct 20, 2026

• Lake: transDeps misses a transitive dependency (and its extern_lib) depending on the order of requires

  bugP-medium
  leanprover/lean4#15798 · Filter by author kmrasmussen opened on Oct 19, 2026

• Partial fixpoints do not generalize arguments properly in a module

  bug
  leanprover/lean4#15777 · Filter by author hgoldstein95 opened on Oct 19, 2026
  1