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Elaboration- and compile-time evaluation of `a <<< b` on literals called the runtime `Nat.shiftLeft`, which aborts the process when `a ≠ 0` and `b` does not fit in 32 bits. Check the new `canEvalNatShiftLeft` before evaluating in `Meta.reduceNat?`, the `Nat`/`Fin`/`BitVec` shift simprocs, `Sym` ground evaluation, the `grind` shift propagators and the LCNF constant folder. Closes leanprover#15193 Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
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This PR stops elaboration-time and compile-time evaluation of
Natleft shifts whose shift amount does not fit in 32 bits, so they no longer abort the Lean process with "Nat.shiftl exponent is too big".The runtime's
lean_nat_shiftlcallslean_internal_panicwhen the shifted value is nonzero and the shift amount is at least2^32. The kernel already refuses such shifts with a kernel exception, andNat.powis protected on the elaborator side bycheckExponent, but several procedures evaluated<<<on literals unconditionally. A new predicate,Lean.canEvalNatShiftLeft a b(a = 0 ∨ b < UInt32.size), is now checked before evaluating a left shift in:Meta.reduceNat?(whnf,rfl,decide)Nat.reduceShiftLeft,Fin.reduceShiftLeft,BitVec.reduceShiftLeft,BitVec.reduceHShiftLeftandBitVec.reduceShiftLeftZeroExtendsimprocsSymground evaluation of shifts onNat,Int,FinandBitVecand ofBitVec.shiftLeft/shiftLeftZeroExtend(cbv,bv_decidenormalization)grindpropagators forNatandBitVecleft shiftsNat.shiftLeftWhen the predicate fails, the shift is left unevaluated, so the affected tactics report an ordinary failure (for
cbv, which then unfoldsNat.shiftLeft, "maximum recursion depth has been reached"). Shifts the runtime can evaluate are unaffected; unlikecheckExponent, no size threshold is introduced.Closes #15193
AI assistance: the bug was found with, and this change and its tests were drafted with, an AI assistant (Claude). The new test and the full stage1 test suite (4219 tests) pass on a local build of this branch (aarch64 Linux).
🤖 Generated with Claude Code