IAM User.fromUserName does not implement IUser functions

[seawatts]

When trying to create an IAM User from @aws-cdk/aws-iam with User.fromUserName the following IUser functions throw an erro.

• addToGroup
• attachInlinePolicy
• addManagedPolicy

https://github.com/aws/aws-cdk/blob/master/packages/%40aws-cdk/aws-iam/lib/user.ts#L145-L154

Reproduction Steps

import { PolicyStatement, User } from '@aws-cdk/aws-iam';

const importedUser = User.fromUserName(this, 'ImportedUser', 'my-user');
importedUser.attachInlinePolicy(new PolicyStatement({...}));

Error Log

Cannot add inline policy to imported User

Environment

• CLI Version : 1.20.0
• Framework Version : 1.20.0
• OS : Mac OS
• Language : Typescript 3.7.0

Other

https://gitter.im/awslabs/aws-cdk?at=5dc31ea914d55a3785e7ca39 (Other people are having this problem)
#3738 (PR implementation of IUser)
#3490 (issue associated with PR)

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This is 🐛 Bug Report

[rix0rrr]

More of a feature request than a bug. We didn't implement this on purpose (apparently).

Can you describe your use case?

[seawatts]

There was a user created manually in my aws account and now I need to add extra policies to that user.

[rangoju]

@rix0rrr , I had the exact same usecase. Do you happen to know the reason why this was on purpose?

[mariusingjer]

We have the exact same scenario: iam user creation is disabled, they opened up for a short time so we could manually create a build user for our service, now I want to configure that build user with CDK.

[github-actions]

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