Rocq Prover before 9.2.0 Guard Checker Accepts Fixpoint Passed as a Higher-Order Argument

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.

Source published

24 Aug 2026 20:08:32 UTC

Source modified

24 Sep 2026 14:18:52 UTC

Observed here

23 Oct 2026 15:48:27 UTC

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CVSS 4.0 · 6.8 CVSS:4.0/AV:L/AC:L/AT:N/PR:N/UI:P/VC:N/VI:H/VA:N/SC:N/SI:N/SA:N

CVSS 3.1 · 6.3 CVSS:3.1/AV:L/AC:L/PR:N/UI:R/S:C/C:N/I:H/A:N