as any unexpectedly breaks function parameter type inference

[haoqunjiang]

Bug Report

🔎 Search Terms

as any
type casting
function parameter
generic type inference

🕗 Version & Regression Information

• This is the behavior in every version I tried, and I reviewed the FAQ for entries about generics and any and type casting

⏯ Playground Link

• Playground link with relevant code

💻 Code

type UserConfig = {
  target: "es5" | "es2015" | "esnext";
  fileName: (format: string) => string;
};
type UserConfigExport = UserConfig | ((env: object) => UserConfig);

function defineConfig<T extends UserConfigExport>(config: T): T {
  return config;
}
defineConfig(() => ({
  target: "es5",
  fileName: (format) => `index.${format === "es" ? "mjs" : "cjs"}`,
}));


defineConfig(({
  target: "es5",
  // Somehow the type inferrence of this function becomes incorrect
  fileName: (format) => `index.${format === "es" ? "mjs" : "cjs"}`,
})) as any;

// Shouldn't error here, either
defineConfig(() => ({
  target: "es5",
  fileName: (format) => `index.${format === "es" ? "mjs" : "cjs"}`,
})) as any;

🙁 Actual behavior

The ones with an as any typecasts failed.

🙂 Expected behavior

All defineConfig calls should pass.

[RyanCavanaugh]

Because T is in the return position, a contextual type on the function call acts as an inference candidate, so having as any here is the same as if you had written

defineConfig<any>(() => ({
  target: "es5",
  fileName: (format) => `index.${format === "es" ? "mjs" : "cjs"}`,
}));

which displays the same behavior.

[haoqunjiang]

Thank you so much for the explanation!
It does make sense once I replace as any with <any>.

I didn't realize the connection because the code came from a refactor of the function from non-generic to generic types 😂