[4.9.2-rc regression] if/else narrowing with a branded string discriminant no longer eliminates matched constituents in later branches

[seansfkelley]

Bug Report

🔎 Search Terms

assert never branded string literal union type narrowing

🕗 Version & Regression Information

This changed between versions 4.8.4 and 4.9.2-rc

⏯ Playground Link

Playground link

💻 Code

type Left = 'left';
type Right = 'right' & { right: 'right' };
type Either = Left | Right;

function assertNever(v: never): never {
  throw new Error('never');
}

declare const value: Either;

if (value === 'left') {
  const foo: 'left' = value;
} else if (value === 'right') {
  const bar: 'right' = value; // correctly narrowed despite branding...
} else {
  assertNever(value); // ...but not eliminated as a candidate
}

Output

"use strict";
function assertNever(v) {
    throw new Error('never');
}
if (value === 'left') {
    const foo = value;
}
else if (value === 'right') {
    const bar = value; // correctly narrowed despite branding...
}
else {
    assertNever(value); // ...but not eliminated as a candidate
}

Compiler Options

{
  "compilerOptions": {
    "strict": true,
    "noImplicitAny": true,
    "strictNullChecks": true,
    "strictFunctionTypes": true,
    "strictPropertyInitialization": true,
    "strictBindCallApply": true,
    "noImplicitThis": true,
    "noImplicitReturns": true,
    "alwaysStrict": true,
    "esModuleInterop": true,
    "declaration": true,
    "experimentalDecorators": true,
    "emitDecoratorMetadata": true,
    "target": "ES2017",
    "jsx": "react",
    "module": "ESNext",
    "moduleResolution": "node"
  }
}

🙁 Actual behavior

The compiler claims the assertNever is reachable (by virtue of only narrowing value to Right in the last branch).

🙂 Expected behavior

The assertNever is considered unreachable (that is, value is narrowed to never).

[ahejlsberg]

This is a regression caused by #51076. Putting up a PR to fix now.