Using declarationMap is causing unexpected errors

Type of Issue

[ x] Bug Report
[ ] Feature Request

(perhaps it’s an unsupported ts feature, and we should threat this as a feature request)

Description

I’ve tried to use the tsconfig property declarationMap for getting sources with generated libraries. (see https://www.typescriptlang.org/docs/handbook/release-notes/typescript-2-9.html#new---declarationmap). Enabling this flag on a project in an angular-cli generated project (either ng 8 or v9.0.0-rc.4), causes the following issues:

ERROR: error TS5069: Option 'declarationMap' cannot be specified without specifying option 'declaration' or option 'composite'.
An unhandled exception occurred: error TS5069: Option 'declarationMap' cannot be specified without specifying option 'declaration' or option 'composite'.

The message indicates that I should add ‘declaration’ or ‘composite’ as well. In the default generated tsconfig.lib.json, the declaration is actually set to true. I’ve tried to enable the ‘composite’ flag as well, but this is causing other issues.

How To Reproduce

• create a new cli workspace (ng new mywebsite)
• create a new project in the workspace (ng g library myproject)
• add the flag declarationMap to projects/myproject/tsconfig.lib.json
• build the library (ng build myproject)

Expected Behaviour

I was expecting that .d.ts.map files would be generated

Version Information

$ node_modules/.bin/ng-packagr --version
ng-packagr: 5.7.1
@angular/*: 8.3.20
typescript: 3.5.3
rxjs: 6.4.0
node: v10.16.0
npm: 6.13.1

[arobinson]

While I understand being busy, this feature is very important for mono-repo development. Without this working, VS code ends up loading the *.d.ts files instead of the source files.

Example layout:

project-root/
  angular-application/
    src/
      foo.ts
  angular-libraries/
    projects/
      some-library/
        src/
          lib/
            bar.ts
  other-local-library/

In the angular-application, to get Ivy working, one cannot use symlinks and tsconfig paths to get angular-application to reference something in one of the libraries since it uses package.json and node_modules. When trying to go to the definition Bar defined in angular-libraries/projects/some-library/src/bar.ts file from angular-application/src/foo.ts file, opened file is angular-application/node_modules/some-library/lib/bar.d.ts which is not helpful at all if you mean to edit the bar.ts source code.

If I understand this right, having the declarationMap working would allow that bar.ts file be loaded instead, making the development experience in a mono-repo much better.

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