TypeScript const assertions raise error in babel

Bug Report

Current Behavior Some const assertions result in errors. Seems like the check introduced in this PR is too strict? Because I didn’t see any errors in VSCode till I started webpack dev server.

Input Code

function foo() {
  const a = getObjectAWithLongAssType();
  const b = getObjectBWithLongAssType();

  return [a, b] as const; // no errors in tsc or vscode
  // Previously: return [a,b] as [typeof a, typeof b]
}

Expected behavior/code VSCode outputs the expected readonly tuple type, so I don’t expect any errors in babel.

Babel Configuration (.babelrc, package.json, cli command)

    "@babel/core": "^7.4.0",
    "@babel/plugin-proposal-class-properties": "^7.4.0",
    "@babel/polyfill": "^7.4.0",
    "@babel/preset-env": "^7.4.2",
    "@babel/preset-react": "^7.0.0",
    "@babel/preset-typescript": "^7.3.3",

Possible Solution

@tanhauhau is there a need for tsCheckLiteralForConstantContext to happen in babel? To me it seems like it’s enough to parse as const expressions as assertions and move on, since TypeScript is actually in charge of ensuring the assertions are valid.

[tanhauhau]

sorry for the late MR 😅

[DanielRosenwasser]

Yeah, as const should be very permissive in the syntax which it parses. None of the semantic restrictions should be applied to Babel if it’s just going to get erased anyway.