Object is possibly undefined after undefined check on readonly property

<!-- 🚨 STOP 🚨 STOP 🚨 STOP 🚨 Half of all issues filed here are duplicates, answered in the FAQ, or not appropriate for the bug tracker. Even if you think you've found a *bug*, please read the FAQ first, especially the Common "Bugs" That Aren't Bugs section! Please help us by doing the following steps before logging an issue: * Search: https://github.com/Microsoft/TypeScript/search?type=Issues * Read the FAQ: https://github.com/Microsoft/TypeScript/wiki/FAQ Please fill in the *entire* template below. --> <!-- Please try to reproduce the issue with the latest published version. It may have already been fixed. For npm: `typescript@next` This is also the 'Nightly' version in the playground: http://www.typescriptlang.org/play/?ts=Nightly --> **TypeScript Version:** 3.7.4, 3.9.2 <!-- Search terms you tried before logging this (so others can find this issue more easily) --> **Search Terms:** Object is possibly undefined undefined readonly callback **Code** ```ts class Test { private readonly testProperty?: { value: number }; public constructor(testProperty: { value: number }) { this.testProperty = testProperty; } public method() { if (this.testProperty) { Promise.resolve().then(() => this.testProperty.value); //Object is possibly undefined } } } ``` **Expected behavior:** testProperty should not be resolved as possibly undefined as this is not possible in this scenario as the property has already been checked and it is marked as readonly. **Actual behavior:** testProperty is possibly undefined, requiring another undefined check. **Playground Link:** <!-- A link to a TypeScript Playground "Share" link which demonstrates this behavior --> https://www.typescriptlang.org/play/#code/MYGwhgzhAEAqCmEAu0DeAoa0AOAnAlgG5hLzS7xgAmA9gHYgCe0pyACrjdvLkowPwAuNNGIgArvGF1xAWwBGPaAF8A3Jhzj5IfMGjB6yXOOBIauABSskHLjz7DUosBKnQZCpcoCUaDViQAC3wIADprW25eZgBeFkQbTii+dSxldA1sLR09WXggmioLXwwsLHwAM2grYLCIpPtGEv8y21kQ+FCKCBoQQnhi8MD4Ogti6BiAPhZa8ITIxtCxSW9VaAB6dYB5eQ…