How can I provide a hand-authored declaration file in a ts project?

Question

I have these files:

• foo.ts
• foo.d.ts (hand-authored)
• bar.ts

I want to use TypeScript to generate the following output

• foo.js
• foo.d.ts (same as my hand-authored one)
• bar.ts
• bar.d.ts

TypeScript ignores my hand-authored foo.d.ts in this case and just generates a .d.ts from foo.ts. How can I get TS to pass through hand-authored .d.ts files when there is a corresponding .ts?