[3.7.1.-rc] Regression: exhaustive switch statements are not exhaustive anymore

[karol-majewski]

TypeScript Version: 3.7.1.-rc

Search Terms: noFallthroughCasesInSwitch, exhaustive, exhaustiveness, switch

Code

interface ProcessEnv {
  NODE_ENV: 'development' | 'production';
}

declare const env: ProcessEnv;

declare function getEnvironmentVariable<K extends keyof ProcessEnv>(key: K): ProcessEnv[K];

const getNodeEnvironment = () => { // 💣 Not all code paths return a value.ts(7030)
  switch (getEnvironmentVariable('NODE_ENV')) {
    case 'development':
      return 'development';
    case 'production':
      return 'production';
  }
};

Expected behavior:

No error. The expression provided to switch evaluates to either 'development' or 'production', so the switch statement is exhaustive.

Interestingly enough, if we assign the expression to a local variable, the function becomes exhaustive.

const getNodeEnvironment = () => { // ✅ No error
  const environment = getEnvironmentVariable('NODE_ENV');

  switch (environment) {
    case 'development':
      return 'development';
    case 'production':
      return 'production';
  }
};

Actual behavior:

A compile-time error for getNodeEnvironment.

Not all code paths return a value.ts(7030)

Playground Link

[fatcerberus]

Duplicate of #34661, it looks like.

[karol-majewski]

You're right, thank you.