A record rest pattern's remainder kept its record brand at runtime, so aspect dispatch ignored its static type

LIMIT-EVALUATION-007

The remainder bound by ..rest carried the scrutinee's runtime brand, so dispatch picked the nominal impl although the spec says the remainder is anonymous.

Lifecycle

resolved

Owner

metel-interpreter

Scope

architecture/spec/evaluation.md#evaluation

Origin

v0.14.0 integration rerun (records x dyn / empty residuals), 2026-10-13

Affects · linked records

spec.types.generics.row-conditional-impls.legality-1language spec rule

limitationKnown limitations ↗

Limitation

The rest binding of a record pattern has the static type {}, but the runtime value keeps the brand of the record it came from, and brand-exact impl dispatch runs first (RFC-0121 section 3). Reproduced against develop at 9cb4c9de, with and without --move-check:

aspect Describe { fun describe(&self) -> String; }
record Point { x: i64, y: i64 }
extend Point: Describe { fun describe(&self) -> String { "point".to_string() } }
extend<row R> { ..R }: Describe where all R: Copy { fun describe(&self) -> String { "copy-row".to_string() } }
fun erase(p: Point) -> {} {
    let { x: _, y: _, ..rest } := p;
    rest
}
fun main() {
    let e: {} := erase(Point { x = 1, y = 2 });
    println(e.describe());    // "point"; a literal `{}` prints "copy-row"
}

Impact

Which impl runs for a {} depends on how the value was built, not on its type, so a function generic over { ..R } can observe a brand its signature does not mention. The same happens through dyn Aspect. Tracked by metel-core#1446 (v0.14.1).

Affects

• spec.types.generics.row-conditional-impls.legality-1

Resolution

Fixed 2026-10-13, metel-core#1446 (v0.14.1). The evaluator built the remainder of a record rest pattern with the scrutinee's type_id (evaluator/pattern/mod.rs); it now builds it with none, matching spec.expressions "Record rest patterns" ("the remainder loses the nominal brand"). The regression fixture evaluator/records/release_matrix_empty_residual_dispatch_follows_static_type is unskipped and also covers a non-empty remainder and dyn Aspect dispatch.