Not sure if it's a bug or a feature (I'll let better specialists decide about that).

But the typical way of testing a component that requires inputs is to actually pass inputs the way they're supposed to be passed, with a test such as this one:

import { async, ComponentFixture, TestBed } from '@angular/core/testing';

import { TestComponent, RequiredInput } from './test.component';
import { Component } from '@angular/core';
import { By } from '@angular/platform-browser';

@Component({
  template: '<app-test [requiredInput]="input"></app-test>'
})
class FixtureComponent {
  input!: RequiredInput;
}

describe('TestComponent', () => {
  let fixtureComponent: FixtureComponent;
  let component: TestComponent;
  let fixture: ComponentFixture<FixtureComponent>;

  beforeEach(async(() => {
    TestBed.configureTestingModule({
      declarations: [ TestComponent, FixtureComponent ]
    })
    .compileComponents();
  }));

  beforeEach(() => {
    fixture = TestBed.createComponent(FixtureComponent);
    fixtureComponent = fixture.componentInstance;
    component = fixture.debugElement.query(By.directive(TestComponent)).componentInstance;
    fixtureComponent.input = { anyKey: 'anyValue'};
    fixture.detectChanges();
  });

  it('should not fail', () => {
    const expectedResult: RequiredInput = { anyKey: 'anyValue' };
    expect(component.requiredInput).toEqual(expectedResult);
  });
});

Not sure if it's a bug or a feature (I'll let better specialists decide about that).

But the typical way of testing a component that requires inputs is to actually pass inputs the way they're supposed to be passed, with a test such as this one:

This "workaround" works for me. Thanks!

I think the main problem (I described) is that the component is created by the TestBed without any parent that gives the input into the component itself. So it could be a way to pass an argument object while creating the component fixture - e.g.:

beforeEach(() => {
  fixture = TestBed.createComponent(TestComponent, {
    requiredInput: { anyKey: 'anyValue'}
  });
  component = fixture.componentInstance;
  fixture.detectChanges();
});

In this case each attribute represents a value the TestBed sets to the same named input of the component as a default value.