Share Component between 2 modules

For completeness, according to Gunter’s answer, use a SharedModule:

SharedModule

import { NgModule } from '@angular/core';
import { CommonModule } from '@angular/common';

@NgModule({
    imports: [
        CommonModule
     ],
    declarations: [
        SharedComponent
    ],
    exports: [
        SharedComponent
    ]
})
export class SharedModule {}

app.module.ts

@NgModule({
    imports: [ChildModule, SharedModule],...
})

child.module.ts

@NgModule({
    imports: [SharedModule]
})