Rocq

Print Assumptions does not report that a definition was produced while universe checking was disabled when that definition reaches the caller through Parameter Inline in a module type. Applying a functor inlines the body of the parameter, and the inlining drops the record that the term was built under Unset Universe Checking, so the resulting constant carries no trace of the unsafe operation. A module implementation can therefore prove False using a universe inconsistency, expose it through an inlined parameter, and have Print Assumptions report the dependent proof as closed under the global context. Because Print Assumptions is the in-process audit used to confirm that a development rests on no unexpected assumptions, a dependency built this way passes that audit while proving arbitrary propositions. The standalone checker coqchk does reject the resulting compiled file. The project records this in dev/doc/critical-bugs.md under non-fixed bugs and rates the risk as moderate when coqchk is not used.

Published: 2026-08-24T20:08:28.079Z
Updated: 2026-10-26T01:23:51.685Z

The guard checker in Rocq Prover does not follow recursive calls made through a fixpoint's own arguments. A fixpoint may pass itself as a higher-order argument to a second fixpoint, which then applies it to a value that is not a subterm of the structural argument. Passing the recursive function to a plain definition is rejected because the checker unfolds the definition and observes the call, but passing it to a fixpoint is accepted because higher-order recursive calls through fixpoint arguments are not tracked. This admits a type that is definitionally equal to its own negation, so self-application produces False in purely definitional code, without tactics, axioms, plugins or unsafe flags, and Print Assumptions reports the result as closed under the global context. Fixed in Rocq 9.2.0.

Published: 2026-08-24T20:08:32.535Z
Updated: 2026-10-26T01:23:56.439Z

The guard checker in Rocq Prover does not recheck the recursive tree representation of an inductive type parameter after that parameter has been changed by transport. A fixpoint may apply a rewrite along an equality between types to its recursive argument, which the guard checker accepts because the inductive type is preserved, while the recursive tree recorded for the parameter is altered. A second fixpoint that calls the first inherits the altered recursive tree without verification, so a call that is not structurally decreasing is accepted as terminating. The resulting non-terminating definition proves that a natural number equals its own successor and therefore False, from which any proposition follows. The demonstration uses two axioms that follow from univalence and are consistent with the calculus of inductive constructions, so the contradiction comes from the guard check rather than from the assumptions. A fix is proposed but not merged.

Published: 2026-08-24T20:08:31.787Z
Updated: 2026-10-26T01:23:55.463Z

Rocq Prover does not restore the universe graph's copy of the universe checking flag when a module that locally disabled the check is closed. Local Unset Universe Checking inside a module is expected to last only until the module ends, and the global flag is restored, but the universe graph keeps its own copy which is left disabled. The two views then disagree: Test Universe Checking reports the check as enabled while the kernel continues to accept universe-inconsistent terms. With the constraint between two universes no longer enforced, Hurkens' paradox applies and yields a proof of False, from which any proposition follows. The proof uses no axioms, plugins or unsafe features once the module has closed, and Print Assumptions reports it as closed under the global context, so neither the assumption audit nor the flag query reflects the actual kernel state. No fix is available.

Published: 2026-08-24T20:08:33.917Z
Updated: 2026-10-26T01:23:58.366Z

The guard checker in Rocq Prover treats a parameter of a nested mutual fixpoint as uniform without examining calls between the different bodies of that fixpoint. find_uniform_parameters in kernel/inductive.ml inspects only self-recursive calls, so when no body calls itself the function concludes that every parameter is uniform. A parameter that grows through a cross-call from one body to another therefore keeps the subterm specification it inherited from the enclosing fixpoint, and a recursive call guarded by that specification is accepted although the argument is not structurally smaller. A non-terminating definition is admitted as structurally decreasing, which yields a term whose value equals its own successor and so a proof of False, from which any proposition follows. The proof requires no axioms, plugins or unsafe flags and Print Assumptions reports it as closed under the global context. Introduced in Coq 8.20 and fixed in Rocq 9.2.0.

Published: 2026-08-24T20:08:30.607Z
Updated: 2026-10-26T01:23:54.511Z