Aspect implementations may not safely widen a method's generic constraints

GAP-DECLARATIONS-002

An implementation's generic constraints must match the aspect method's exactly, even where a looser signature would be safe.

Lifecycle

planned

Owner

language

Scope

reference/spec/declarations.md#spec.declarations.aspects.implementing-an-aspect.legality-13

Origin

metel-core#1217 limitation analysis; fixtures typechecking/aspects/stage21_neg_08 and stage21_neg_09

Affects · linked records

spec.declarations.aspects.implementing-an-aspect.legality-13language spec rule

RFC-0129rfc

RFC-0149rfc

gapKnown gaps ↗

Gap

An implementation method's generic-constraint conjunction must be structurally equal to the aspect method's, after normalization. This is the RFC-0129 minimal interim rule, and it rejects some implementations that would be sound: one that drops a generic bound (a constraint widening), and one that drops the record kind. Both are rejected with T0012.

Impact

An implementation cannot be more permissive than its aspect's method signature even where that is safe. The workaround is to restate the aspect's constraints exactly.

Affects

• spec.declarations.aspects.implementing-an-aspect.legality-13
• RFC-0129
• RFC-0149

Resolution

Planned for v0.21.1: RFC-0149 (aspect method constraint domain inclusion, letting an implementation weaken a constraint) is under review, tracked as metel-core#895. The two negative fixtures pin the current behaviour and flip to positive fixtures when it lands.