leanprover/lean4

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Lean 4 programming language and theorem prover

Stars: 9.2k
Forks: 990
Open beginner issues: 0
Indexed issues: 1k
Avg merge: 1d 17h
Merged PRs (30d): 175

Dominant language: Lean
License: Apache-2.0
Last GitHub push: Sep 19, 2026
Latest indexed: Sep 20, 2026
Contributing guide: Contributing guide
Code of conduct: No code of conduct
Beginner labels: help wanted

Beginner friendly Everything

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1,007 open issues indexed

RFC: safe withExclusive wrapper around isExclusiveUnsafe Open

RFC

Difficulty 5/5 Over a week Newbie friendliness 35/100

leanprover/lean4#15235 · Opened Sep 20, 2026
`#print axioms` / `collectAxioms` under-reports axioms of imported inductives (in-progress sentinel cached by `exportedAxiomsExt`) Open

bug

Difficulty 4/5 3-5 days Newbie friendliness 48/100

leanprover/lean4#15226 · Opened Sep 19, 2026
`@[deprecated]` on a structure field projection does not warn when constructing the field Open

bug

Difficulty 4/5 3-5 days Newbie friendliness 48/100

leanprover/lean4#15203 · Opened Sep 17, 2026
`simpDiscrCtor?` is broken in the presence of non-type parameters Open

bug

Difficulty 4/5 3-5 days Newbie friendliness 48/100

leanprover/lean4#15200 · Opened Sep 17, 2026
`simp [...]` with a left arrow (`←`) on a simproc should error Open

bug

Difficulty 2/5 1-3 hours Newbie friendliness 84/100

leanprover/lean4#15197 · Opened Sep 17, 2026
`#guard_msgs` hides error messages from commands outside of it Open

bug

Difficulty 4/5 3-5 days Newbie friendliness 45/100

leanprover/lean4#15196 · Opened Sep 17, 2026
`bv_decide` fails with `unknown free variable '_fvar.126'` under certain contexts Open

bug

Difficulty 4/5 3-5 days Newbie friendliness 48/100

leanprover/lean4#15195 · Opened Sep 17, 2026
`Nat.shiftLeft` by 2^32 or more crashes Lean in `rfl`, `decide`, `simp`, `cbv`, `grind` and the compiler Open

Difficulty 4/5 3-5 days Newbie friendliness 45/100

leanprover/lean4#15193 · Opened Sep 17, 2026
Emscripten build passes `-pthread` unconditionally, ignoring `MULTI_THREAD=OFF` Open

Difficulty 3/5 1-2 days Newbie friendliness 72/100

leanprover/lean4#15186 · Opened Sep 16, 2026
grind explodes on a simple example with `Array.range` Open

bug

Difficulty 4/5 3-5 days Newbie friendliness 52/100

leanprover/lean4#15183 · Opened Sep 16, 2026
(kernel) application type mismatch in `cbv` with `ite_true` Open

bug

Difficulty 3/5 1-2 days Newbie friendliness 55/100

leanprover/lean4#15182 · Opened Sep 16, 2026
MULTI_THREAD=OFF build fails: std::adopt_lock_t used in thread.h without including <mutex> Open

Difficulty 2/5 1-3 hours Newbie friendliness 86/100

leanprover/lean4#15172 · 1 comment · Opened Sep 15, 2026
perf: reduce Nat.powMod kernel replay cost Open

Difficulty 4/5 3-5 days Newbie friendliness 52/100

leanprover/lean4#15166 · Opened Sep 15, 2026
RFC: structured log entries in Lake, and a JSON log format for `lake build` Open

RFC

Difficulty 5/5 Over a week Newbie friendliness 35/100

leanprover/lean4#15139 · 2 comments · Opened Sep 12, 2026
RFC: Support dependency in Verso docstrings Open

RFC

Difficulty 5/5 Over a week Newbie friendliness 45/100

leanprover/lean4#15137 · 2 reactions · Opened Sep 11, 2026
custom semantic highlighting appears only when source file is first edited Open

bug

Difficulty 4/5 3-5 days Newbie friendliness 52/100

leanprover/lean4#15118 · 6 comments · Opened Sep 11, 2026
#guard_panic misclassifies ordinary errors as panics via a naive substring check, silently deleting the message log Open

bug

Difficulty 3/5 1-2 days Newbie friendliness 68/100

leanprover/lean4#15117 · Opened Sep 11, 2026
`#eval` of an `IO` action aborts `lean` (uncaught `lean::exception`, exit 134) when it reaches a non-`meta` import through a local definition Open

bug

Difficulty 4/5 3-5 days Newbie friendliness 52/100

leanprover/lean4#15113 · 2 comments · Opened Sep 10, 2026
Linux: libleanshared.so exports a partial _Unwind_* ABI, breaking C++ exceptions inside FFI libraries built against libstdc++ Open

Difficulty 4/5 3-5 days Newbie friendliness 45/100

leanprover/lean4#15112 · 1 reaction · Opened Sep 10, 2026
Missing goal state after `grind =>`, `sym =>` and `impossible by` Open

bug

Difficulty 3/5 1-2 days Newbie friendliness 70/100

leanprover/lean4#15108 · 1 comment · 2 reactions · Opened Sep 10, 2026
omega hits the recursion limit on a reflexive multiplication goal with an unused hypothesis Open

bug

Difficulty 3/5 1-2 days Newbie friendliness 64/100

leanprover/lean4#15102 · Opened Sep 9, 2026
`grind` failing to apply lemma if instance is not syntactically equal Open

bug

Difficulty 4/5 3-5 days Newbie friendliness 55/100

leanprover/lean4#15082 · 1 comment · Opened Sep 9, 2026
RFC: use `Std.Refl`/`Std.Symm` instances in `rfl`/`symm` Open

RFC

Difficulty 5/5 Over a week Newbie friendliness 39/100

leanprover/lean4#15081 · 2 reactions · Opened Sep 9, 2026
`deriving instance CoeFun/CoeSort` generates opaque aux constant for reducible type synonyms Open

bug

Difficulty 4/5 3-5 days Newbie friendliness 55/100

leanprover/lean4#15065 · 1 comment · 3 reactions · Opened Sep 8, 2026
Std.Http minor cleanups Open

RFC

Difficulty 4/5 3-5 days Newbie friendliness 35/100

leanprover/lean4#15051 · 1 reaction · Opened Sep 7, 2026
No `goalsAccomplished` message is produced for theorems generated using `elab` Open

bug P-medium

Difficulty 3/5 1-2 days Newbie friendliness 72/100

leanprover/lean4#15044 · 1 comment · Opened Sep 6, 2026
Delaborator omits an instance binder's name when it is used only in a later binder's type Open

P-medium

Difficulty 2/5 1-3 hours Newbie friendliness 78/100

leanprover/lean4#15037 · Opened Sep 4, 2026
`pp.all` output does not round-trip when a local's type has `optParam`/`autoParam` binders Open

P-low

Difficulty 4/5 3-5 days Newbie friendliness 55/100

leanprover/lean4#15035 · Opened Sep 4, 2026
`Failed to find LCNF signature` reported to users where v4.20 gave the `noncomputable` hint Open

P-low

Difficulty 4/5 3-5 days Newbie friendliness 45/100

leanprover/lean4#15034 · Opened Sep 4, 2026
Compiler PANIC in `ExplicitBoxing` when `deriving Repr`/`BEq` over an `unsafe instance` Open

P-medium

Difficulty 4/5 3-5 days Newbie friendliness 52/100

leanprover/lean4#15033 · 1 comment · Opened Sep 4, 2026
`Nat.lt` docstring Open

P-low

Difficulty 1/5 Under an hour Newbie friendliness 85/100

leanprover/lean4#15014 · Opened Sep 4, 2026
Very high CPU usage when making any edits at the top of a file Open

bug P-medium

Difficulty 4/5 3-5 days Newbie friendliness 45/100

leanprover/lean4#15013 · 4 comments · 3 reactions · Opened Sep 4, 2026
typecheck error sometimes escapes `first` Open

bug P-medium

Difficulty 4/5 3-5 days Newbie friendliness 45/100

leanprover/lean4#15000 · 1 reaction · Opened Sep 2, 2026
stack overflow hangs instead of aborting when the fault lands inside the allocator Open

bug P-medium

Difficulty 4/5 3-5 days Newbie friendliness 25/100

leanprover/lean4#14992 · Opened Sep 1, 2026
olean loader: no-mmap fallback re-reads the entire file after the header probe Open

bug P-low

Difficulty 3/5 1-2 days Newbie friendliness 72/100

leanprover/lean4#14991 · Opened Sep 1, 2026
Private proof materializes a public equation theorem Open

bug P-medium

Difficulty 4/5 3-5 days Newbie friendliness 45/100

leanprover/lean4#14986 · Opened Sep 1, 2026
`subst` and `simp` reorder hypotheses when rewriting Open

bug P-low

Difficulty 4/5 3-5 days Newbie friendliness 52/100

leanprover/lean4#14985 · 1 reaction · Opened Sep 1, 2026
Meta issue: Lean Kernel Logic renovation Open

P-high

Difficulty 5/5 Over a week Newbie friendliness 25/100

leanprover/lean4#14977 · 1 comment · 7 reactions · Opened Aug 31, 2026
Unexpected compilation error "Could not find native implementation of external declaration" Open

bug P-medium

Difficulty 4/5 3-5 days Newbie friendliness 52/100

leanprover/lean4#14975 · Opened Aug 30, 2026
Emscripten libuv stub definitions do not match their declarations Open

P-medium

Difficulty 2/5 1-3 hours Newbie friendliness 84/100

leanprover/lean4#14973 · 1 comment · Opened Aug 30, 2026
kernel rejects simp proof, dischargeEqnThmHypothesis not well behaving Open

bug

Difficulty 4/5 3-5 days Newbie friendliness 46/100

leanprover/lean4#14961 · 1 comment · Opened Aug 29, 2026
deriving ToJson/FromJson generates ill-typed code for self-recursion in a non-first mutual member Open

Difficulty 2/5 1-3 hours Newbie friendliness 85/100

leanprover/lean4#14958 · Opened Aug 28, 2026
Cannot derive eq_def for an @[irreducible] well-founded definition Open

bug P-low

Difficulty 4/5 3-5 days Newbie friendliness 45/100

leanprover/lean4#14957 · 5 comments · Opened Aug 28, 2026
Unification sees through type synonyms when checking instances' types Open

bug

leanprover/lean4#14949 · 1 assignee · Opened Aug 28, 2026
RFC: Mutual inductives with heterogeneous universes Open

RFC

Difficulty 5/5 Over a week Newbie friendliness 25/100

leanprover/lean4#14944 · 4 comments · Opened Aug 27, 2026
`Std.Http`: `URI.Query` lookups compare percent-encoded bytes, which are not canonical Open

bug

Difficulty 3/5 1-2 days Newbie friendliness 76/100

leanprover/lean4#14934 · 1 reaction · Opened Aug 26, 2026
RFC: Generalize Array.binSearch with partition-point-style API Open

RFC

Difficulty 5/5 Over a week Newbie friendliness 35/100

leanprover/lean4#14931 · Opened Aug 26, 2026
simp still panics with appArg!/appFn! in the non-indexed rewriting path Open

Difficulty 2/5 1-3 hours Newbie friendliness 82/100

leanprover/lean4#14930 · 1 comment · Opened Aug 25, 2026
Selectable.one + TCP.recvSelector retains RSS on repeated wait-for-read (not recv?) Open

bug P-medium

Difficulty 4/5 3-5 days Newbie friendliness 48/100

leanprover/lean4#14924 · Opened Aug 25, 2026
Std.Http.Server.serve accept loop leaks RSS: ContextAsync `while` chains Tasks until shutdown Open

bug P-medium

Difficulty 4/5 3-5 days Newbie friendliness 52/100

leanprover/lean4#14918 · Opened Aug 24, 2026

leanprover/lean4

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