feat: Variadic concat#7231
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`concat` now accepts 2 or more sequence arguments when running under `ClarityVersion::Clarity6`. The 2-argument case is byte-for-byte equivalent to the previous v205 implementation, so older-version contracts running at Epoch 4.0 see no behavior change. The variadic form is folded left-to-right, charging the v205 cost formula on every fold step. This means `(concat a b c)` costs the same as the equivalent `(concat (concat a b) c)` — no arbitrage between the two syntactic forms — while still benefiting from a single allocation path at runtime. Implementation: - Runtime: new `special_concat_v600` in `functions/sequences.rs`; `special_concat` now dispatches by epoch (v200 for 2.0, v205 for 2.05–3.4, v600 for 4.0+). - Type checker: `check_special_concat` switches to `check_arguments_at_least(2)` for Clarity 6+ and folds `least_supertype` / `max_len` across all args. The 2-arg path is preserved exactly for older versions. Tests cover variadic over buff, string-ascii, and list; arity errors (0 and 1 arg); mixed-type rejection at both runtime and analysis time; and a regression check that pre-Clarity-6 contracts still reject 3-arg `concat`. Closes stacks-network#7112 Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
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Description
Allow
concatto take variable arguments. This allows much cleaner and more efficient buffer building than in Clarity 5:Applicable issues
Additional info (benefits, drawbacks, caveats)
This change means that buffer building costs now scale linearly with the number of arguments, rather than quadratically, since we can eliminate doing a buffer allocation for each argument. We may wish to update the costs contract to reflect the changes
Checklist
docs/property-testing.md)changelog.d/README.md)rpc/openapi.yamlfor RPC endpoints,event-dispatcher.mdfor new events)clarity-benchmarkingrepo