[api-extractor] handle merged (augmented) symbols properly

[shlomiassaf]

When declaration merging is applied on an imported type a new symbol is created (merged symbol).

This symbol contains SymbolFlags.Transient which does not have a matching AstSymbol which in turn throws an error

import { RowContext } from '@angular/cdk/table';
export interface MyExtender extends RowContext<any> {}

declare module '@angular/cdk/table/typings/table.d' {
  interface RowContext<T> {
      myAugmentedProp: string;
  }
}

The type itself is fine, it is just a merged type and the transient check should not stop it, for this particular situation.

This is my current workaround, i'm not sure it covers all scenarios:

import { AstSymbolTable } from '@microsoft/api-extractor/lib/analyzer/AstSymbolTable';

const { _fetchAstSymbol } = AstSymbolTable.prototype as any;

(AstSymbolTable.prototype as any)._fetchAstSymbol = function (followedSymbol, addIfMissing, astImportOptions, localName) {
  let result = _fetchAstSymbol.call(this, followedSymbol, addIfMissing, astImportOptions, localName);

  if (!result && astImportOptions && followedSymbol.flags & ts.SymbolFlags.Transient) {
    followedSymbol.flags = followedSymbol.flags ^ ts.SymbolFlags.Transient;
    result = _fetchAstSymbol.call(this, followedSymbol, addIfMissing, astImportOptions, localName);
    followedSymbol.flags = followedSymbol.flags | ts.SymbolFlags.Transient;
  }

  return result;
};

[octogonz]

Could you provide a branch with a simple project that repros this error? I have never encountered transient symbols in any of our projects at work, so I'm pretty unfamiliar with this feature (or why anyone would want it heheh). Thanks!

[shlomiassaf]

@octogonz I'll do that shortly...

Augmenting types is quite common, especially with plugin architecture and type lookup maps.

Anyway, the transient type is created by TS and represents the merged (union) type of the original type and the augmentation type...

[octogonz]

Does the transient type have the same signature for every source file that references it? Or does it create a special extended signature that is only visible in the source file that contains the merged declaration? In the latter case, API Extractor would need to somehow distinguish the two different RowContext signatures, and have different names for them.

[octogonz]

Closing, since this is a duplicate of #1709 which has made more progress.