A file exports at least one real (non-type-only, non-re-export) symbol but no corresponding test file exists anywhere in the project.

// format-currency.ts — exported behavior with no test file anywhere
export function formatCurrency(cents: number): string {
  return `$${(cents / 100).toFixed(2)}`;
}

Not flagged — the file only re-exports (a barrel) or only exports types, so there's no behavior of its own to test:

// index.ts
export * from "./format-currency";
export type { CurrencyOptions } from "./types";

Matching: for each candidate source file, the check substitutes {name} (the file's stem) into every test_match_patterns template, resolves the result relative to the source file's own directory, and looks for it in the project's known-files universe (built from the import/export graph, same construction Design.ImportFanOutOutlier uses). The default templates cover same-directory and sibling-__tests__/ conventions for both .test. and .spec. naming, across .ts/.tsx/.js/.jsx/.mjs/.mts/.cts extensions. Configure test_match_patterns for a project with a different layout (e.g. a top-level tests/ directory mirroring src/).

Exemptions: a file already recognised as a test/mock itself (test_file_patterns, same default list as Design.OrphanExport) is never flagged for missing its own test. A framework entry-point file (framework_entry_patterns, shared default with Design.OrphanExport/Warning.UnusedImport) is exempt too — the framework runtime, not a unit test, is what exercises it.

Scope: the known-files universe only includes files that appear as an import source, an export site, or a resolved import target — a test file with zero imports and zero exports of its own wouldn't be visible here even if it exists on disk. In practice a test file almost always imports the module it's testing, so this is a narrow gap. A project with a tests/ directory that mirrors src/ at the project root (rather than sitting alongside each source file) needs a custom test_match_patterns entry — the defaults only check same-directory and sibling-__tests__/ locations. The default templates aren't matched against the source file's own extension — they're just a flat list covering every supported extension — so a .ts source with only a stray .test.js sibling would technically also suppress a finding; this cross-extension false-negative is accepted as harmless in practice.