Ionic 5 was announced a few hours ago (12th Feb 2020) and I upgraded my one of the small production app to Ionic 5 along with Angular 9:

# To update to Ionic 5
npm install @ionic/angular@latest @ionic/angular-toolkit@latest --save-exact --save

# To update to Angular 9
ng update @angular/core @angular/cli

But when I did ionic serve, I started getting the bellow error:

Error: Angular JIT compilation failed: '@angular/compiler' not loaded!
  - JIT compilation is discouraged for production use-cases! Consider AOT mode instead.
  - Did you bootstrap using '@angular/platform-browser-dynamic' or '@angular/platform-server'?
  - Alternatively provide the compiler with 'import "@angular/compiler";' before bootstrapping.
    at getCompilerFacade (core.js:610)
    at Function.get (core.js:16065)
    at getInjectableDef (core.js:362)
    at injectableDefOrInjectorDefFactory (core.js:16816)

I came across a few Angular GitHub issues:

https://github.com/angular/angular-cli/issues/16873
https://github.com/angular/angular/issues/32466

They are saying to include import '@angular/compiler'; in main.ts file but when I matched one of my other Angular 9 application (which I updated recently), I don't see such configurations there.

Is Angular 9 not compatible with Ionic 5?

Updated & Correct Solution to fix this

Based on the answer from Tran Quang, I went to see the CHANGELOG.md of ionic-native and came to know that they recently updated their package to compile with Angular 9.

Hence you need to update any/all dependencies of ionic-native. For this, look at all the dependencies in your package.gson file which start with @ionic-native/ and update them one by one.

For example, this is my package.gson:

So I'll have to run the following commands to update all my @ionic-native dependencies:

npm i @ionic-native/core@latest
npm i @ionic-native/camera@latest
npm i @ionic-native/firebase-x@latest
npm i @ionic-native/splash-screen@latest
npm i @ionic-native/status-bar@latest

Same you have to do for your @ionic-native dependencies. Just make sure, those are updated minimum to v5.21.5 (because a few old releases were not working).

Cheers