Generic Coercion Evidence

This document owns the compiler architecture for structural where Source as Target evidence. Concrete declaration selection, conversion plans, ownership, and lowering remain owned by Borrow Coercions. Public syntax and behavior belong in spec/22-borrow-coercions.md.

Predicate Identity

The requirement has a dedicated AST node containing canonical source and target types plus the authored source, exact as, and target spans. It is not encoded as type equality or an operator: the right side already states the result, and coercion capability and visibility rules are independent of operator selection.

Validation accepts &T or &+T for one visible generic parameter and a borrowed target. A readonly source cannot promise a readwrite target. Qualification traverses both sides in the declaring source context, duplicate detection uses canonical type identity, and formatter, JSON, semantic tokens, hover, and type occurrences consume the node directly.

Delayed Authority

TypeEnvironment stores coercion evidence separately from nominal, copy, equality, index, and expansion evidence. Contextual conversion, explicit as, receiver-method fallback, comparisons, and indexing all call the ordinary coercion selector with that environment. A requirement match produces the same selected conversion shape as a concrete declaration but records the authored requirement span as temporary authority.

After generic substitutions are known, call specialization resolves the concrete source type across its owning source contexts and reruns the ordinary declaration selector for the exact target. The resulting immutable plan replaces requirement authority with the real declaration span, callable target, receiver capability, and owner substitutions before lowering. A missing or inaccessible concrete declaration is diagnosed at the generic call.

No runtime witness, requirement table, nominal-name rule, source-text comparison, or second coercion algorithm exists. Once specialized, ownership, provenance, regions, editor navigation, and IR consume the ordinary concrete plan.