Bug Report

TypeScript doesn't understand that array shift mutates the values of the array.

https://www.typescriptlang.org/play?#code/MYewdgzgLgBAHjAvDA2gRgDQwExYMwC6A3AFAkCWAZjABRwoAMBSiyaAlDAN4kzwB0EABZUoNdqT5Va9Ji2TZOPPn1CQQAGwCm-DSADmNAOQgA1kYm8YAXxLWgA

🔎 Search Terms

Array
shift
unshift

🕗 Version & Regression Information

Happens on nightly, currently failing on 4.7.0 also.

⏯ Playground Link

https://www.typescriptlang.org/play?#code/MYewdgzgLgBAHjAvDA2gRgDQwExYMwC6A3AFAkCWAZjABRwoAMBSiyaAlDAN4kzwB0EABZUoNdqT5Va9Ji2TZOPPn1CQQAGwCm-DSADmNAOQgA1kYm8YAXxLWgA

💻 Code

const x: number[] = [1, 2, 3];
if (x[0] === 1) {
  x.shift();
  if (x[0] === 2) {    // <--- Error here.
    console.log('ok');
  }
}

🙁 Actual behavior

Error evaluating x[0] after a shift. TypeScript thinks array values are fixed to the values from initialization.

🙂 Expected behavior

Error evaluating x[0] is typed as 'number', not a numeric literal (i.e. no error in example, console statement is reachable and prints).