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Strict mode: Object is possibly 'null' when it really isn't #16285

Description

@zaggino

TypeScript Version: 2.3.4

Code

/*
tsconfig.json
{
  "compilerOptions": {
    "target": "ES2015",
    "module": "commonjs",
    "strict": true
  }
}
*/

let maybeNumber = Math.random() > 0.5 ? 2 : null;
let ones = [1, 1, 1];
let twoes;
if (maybeNumber) {
  twoes = ones.map(x => x * maybeNumber);
}
console.log(maybeNumber, twoes);

Expected behavior:
Compilation passes, TS should know that .map function will be executed immediately and so maybeNumber will not be null

Actual behavior:
Compilation error: error TS2531: Object is possibly 'null'

Weird:
When I change let maybeNumber to const maybeNumber compilation passes for some reason, even though type of maybeNumber should be still number | null

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