Support arrays, structs, and identifiers in ternary operators

[mohammadfawaz]

Summary

Extends the ternary instruction to accept array, struct, and identifier operands (in addition to the pre-existing literal set), gated behind ConsensusVersion::V15.

• console and circuit: new Ternary impls for Literal and Plaintext (array/struct aware, recursive on fields/elements); new Ternary impl for IdentifierLiteral.
• synthesizer-program: ternary operation now handles non-literal operands; type-checking ensures both branches have matching array shape / struct layout.
• synthesizer: cost model updated; deployment validation via check_no_non_literal_ternary rejects non-literal / non-pre-V15-supported ternary before V15.

Consensus gating

Before V15, deployments containing ternary on arrays, structs, or identifier operands are aborted — in function, finalize, closure, and constructor bodies. At V15 and later, they are accepted and executable.

String is intentionally not supported

The string literal type is not a permitted ternary operand at any version:

• Pre-V15: check_no_non_literal_ternary's pre-V15 allow-list excludes string (it was never in the original macro-based list).
• V15+: the new ensure_no_string_leaves check in Ternary::output_types recursively walks the branch plaintext type and rejects any string leaf (including inside arrays or structs, local or external).

Rationale: Aleo strings are variable-length byte arrays, but the byte-wise MUX that underlies circuit ternary requires both branches to have matching byte-lengths. During deployment, circuit synthesis samples strings with Literal::sample(String, rng), which yields random lengths; supporting strings would require either a padded sampling strategy or a graceful length-check path, neither of which is a small change. If we ever want strings, it would be a follow-up with its own consensus gate.

Tests

synthesizer/src/vm/tests/test_v15/ternary_plaintext.rs covers:

• V14 rejection / V15 acceptance for array, struct, identifier, finalize, closure, and constructor cases.
• Execution: arrays of various sizes, nested structs, identifiers, structs containing external records.
• Type-mismatch rejection (array size mismatch, struct field mismatch).
• ternary inside finalize.
• String rejection at V15 (both plain string and structs containing a string field).

Unit tests in console/program/src/data/literal/ternary.rs and circuit/program/src/data/literal/ternary.rs exercise Literal::ternary dispatch across all supported variants, including Identifier.

[mohammadfawaz]

Support for this in Leo is here: ProvableHQ/leo#29349

Shows on average 7.8% drop in Aleo program sizes across the Leo test suite.