fix: stop overriding tsconfig's preserveSymlinks

[dayongkr]

Opened #405 as a question. I think this is the right direction, so here is the change alongside it.

What

DEFAULT_OPTIONS is spread after the user's tsconfig in getCompilerOptions:

compilerOptions: {
  ...options,          // tsconfig
  ...compilerOptions,  // DEFAULT_OPTIONS + plugin overrides
}

So preserveSymlinks: true was not a default. It was unsettable from tsconfig.

This removes the key rather than setting it to false. Hardcoding false keeps the same defect mirrored: anyone who needs true for npm link would be silently overridden. Removing it restores tsconfig as the source of truth and falls back to the TypeScript default (false), which fixes pnpm out of the box.

The old comment looks backwards

// Ensure TS2742 errors are visible is the opposite of what happens. Forcing it on hides the failure and emits invalid output. With it unset, TypeScript resolves the package and reports the portability error (TS2883).

Test

tests/testcases/externals-pnpm-layout reproduces pnpm's layout: pkg-a is symlinked into a store directory where its transitive pkg-b sits as a sibling, reachable only through realpath.

	emitted
preserveSymlinks: true (before)	declare const value: any;
unset (after)	declare const value: number;

Nothing is reported in the first case. The resolution failure is inside pkg-a's .d.ts, which skipLibCheck: true skips, so noEmitOnError has no diagnostic to act on.

Full suite passes.

Compatibility

Breaking for symlinked packages. A linked package resolving peer dependencies against its consumer (npm link) relied on the forced true and now needs it declared:

{ "compilerOptions": { "preserveSymlinks": true } }

Probably worth a major. Happy to adjust.

Downstream has been working around this for a while. unbuild passes preserveSymlinks: false into this plugin citing #143, and tsup carries a copy of DEFAULT_OPTIONS with the flag flipped.

Refs #405, and #143 which reported this in 2021.