Structural source members

declare function open(path: string, options: { flags?: string; highWaterMark?: number }): void;
const options = { highWaterMark: 3 };
open("output.txt", options);

The two highWaterMark properties have different symbols. An identity query cannot prove their structural correspondence. After obtaining the checked call and selected parameter, a target uses the public source boundary:

const semantics = source.semantics.forNode(callNode);
const call = semantics.operations.call(callNode);
const relation = semantics.types.structuralMembers(
  call.sourceArguments[1].type,
  call.sourceSelectedSignatureParameters[1].selectedType,
);

An available result records each destination property in destination order. Each row is either present, with both exact property symbols, root symbols, effective selected types, optional flags and read/accessor declarations; or absent, for an optional property not found on the source type. An optional source property is still present: its runtime presence must be handled by the target. This differs from an absent property and from a present property whose value is undefined. Extra source properties are not projected.

The shared owner resolves the destination symbol’s compiler key through TSTS’s public property query and joins by the resulting source symbol, not by target-side string comparisons. Generic and mapped property types are the checker’s instantiated property types. Results are cached per source/destination type pair and immutable; compiler type, symbol and syntax handles retain their existing identity contracts.

This is member correspondence, not an assignability test. The accepted call/assignment remains the source of assignment evidence. Index signatures, call signatures and construct signatures are returned separately for both types; a member map does not discharge those obligations or select a callable overload. Generic member types remain compiler types, not proof of native closure. Unions must be narrowed or selected first. Unresolved types, missing required properties, inconsistent member evidence and write-only source accessors return explicit unavailable reasons.

No expression is evaluated by this query. Native snapshot construction, reading an accessor at the correct time, evaluating the source receiver once, preserving object identity and rejecting unsupported native conversions remain target work. There is no target-specific field matching or new checker state.