Function called with spread operator doesn't typecheck even when the length of array and types of arguments are known at compile time.

[alankila]

TypeScript Version: 2.8.0-dev.20180222.

Search Terms:

I've looked for anything mentioning spread and type inference. I did not find this precise issue mentioned anywhere, which is why I'm adding it now.

Code

Save the following file as test.ts, then compile with tsc test.ts.

// A *self-contained* demonstration of the problem follows...
// Test this by running `tsc` on the command-line, rather than through another build tool such as Gulp, Webpack, etc.
    function calledWithSpread(x: string, y: number, z: boolean) {
    }

    let x: [string, number, boolean] = ["foo", 1, false];
    calledWithSpread(...x);
    calledWithSpread(x[0], x[1], x[2]);

Expected behavior:
Since TypeScript is knows the variadic type of x, I would now expect that the spread operator to take that into account and realize that x would contain 3 values rather than some unknown number. Only the latter form currently passes type check.

Actual behavior:
test.ts(5,5): error TS2556: Expected 3 arguments, but got 0 or more.

Playground Link:

Related Issues:
This issue is probably also be covered in the variadic generics work in #5453. However, I expect that resolving just this issue would be easier than the full solution of that bug.

Duplicate of #4130

[typescript-bot]

Automatically closing this issue for housekeeping purposes. The issue labels indicate that it is unactionable at the moment or has already been addressed.