Builtins that have a pkg.Schema argument evaluate this argument as an expression, possibly unifying it with something else. This may result in a cycle. In order to detect the cycle, the cycle information obtained from using the argument must passed to further evaluation. We do this by making the Schema method call CallContext.Expr, which, in turn, uses EvaluatedKeepState to accumulate cycle information in the OpContext. The functions that are passed the OpContext then need to use adt.Unify instead of Value.Unify to be able to detect cycles. NOTE: this strongly hints that we need to change the public API to also be centered around an operation context. This has come up in various situations already. This change requires pkg/gen.go to also pass an OpContext for the generated stubs. We take a somewhat unusual approach here: rather than changing the functions to take an OpContext, we instead call a non-exported function with the OpContext added as the first argument whenever an Exported function with a pkg.Schema argument is found. The reason for this is that we currently piggyback on godoc to generate documentation for the builtins. Adding the OpContext in this signature would be confusing for the the user, as it should not be populated in CUE. Note some counter error were introduced by the new tests. The error messages have some additional or longer paths as a result of this change, which is either a benign change or desirable. Fixes #3649 (fixes derivative issues, was already closed) Signed-off-by: Marcel van Lohuizen <mpvl@gmail.com> Change-Id: I4b5a31aeb5501ec03915d069f8b92f56311124dd Reviewed-on: https://review.gerrithub.io/c/cue-lang/cue/+/1208702 Unity-Result: CUE porcuepine <cue.porcuepine@gmail.com> TryBot-Result: CUEcueckoo <cueckoo@cuelang.org> Reviewed-by: Matthew Sackman <matthew@cue.works>