type FailsType = {
  foo: boolean;
};
type WorksType = { 
  foo: string
};
class TestClass<Param> {
  testFunc: <T extends Partial<Param>>(
    param: T
  ) => {testProp?:Param extends Partial<T> ? 'yes' : 'no'} = (param)=>({});
}
type inlineTest = FailsType extends Partial<FailsType> ? 'yes' : 'no';
const inlineTestValue:inlineTest = 'yes';

// Here i use the Works Type
const testClassWorks = new TestClass<WorksType>();
const resultWorks = testClassWorks.testFunc({ foo: 'works' });
resultWorks.testProp='yes';

// Here i use the Fails Type
const testClassFails = new TestClass<FailsType>();
const resultFails = testClassFails.testFunc({ foo: false });
resultFails.testProp='yes';