Schematic with multi select x-promt errors when using command line parameter: 'path ".foo" should be array'

Start with the schematics CLI option-processing path named in the stack trace, schema-option-transform.js, and reproduce the failure using the linked minimal reproduction repository. Compare prompt input with the --name command-line input for single and multiple selections. Done means both command-line forms pass schema validation without the reported array error.

🐞 Bug report

Command (mark with an x)

• new
• build
• serve
• test
• e2e
• generate
• add
• update
• lint
• xi18n
• run
• config
• help
• version
• doc

Is this a regression?

No, it's not. Multi select prompt is just supported within angular cli > 9.0.0-rc.3.

Description

Creating an angular schematic that prompts the user for multiple options is possible since https://github.com/angular/angular-cli/issues/16104. The prompt works quite good now but using the command line for multi selections will cause an error:

Error: Schematic input does not validate against the Schema: {"name":"paul"}
Errors:

  Data path ".name" should be array.
...

At first I opened an issue for the docs because I thought it's just not documented: https://github.com/angular/angular/issues/33851 But after checking the implementation of the angular guard schematic which has --implements parameter (which works) and creating the minimal reproduction repo (see underneath), I think it's a bug somewhere in @angular-devkit/schematics-cli.

🔬 Minimal Reproduction

I created a very minimal repo for reproduction:
https://github.com/d-koppenhagen/ngx-multi-select-schematic

npm i -g @angular-devkit/schematics-cli@next
git clone git@github.com:d-koppenhagen/ngx-multi-select-schematic.git
cd ngx-multi-select-schematic
npm i

✅ Using prompt works good:

schematics .:hello --debug=false
? Which interfaces would you like to implement? (Press <space> to select, <a> to toggle all, <
i> to invert selection)
❯◉ hans
 ◉ peter
 ◉ paul
schematics .:hello --debug=false --name="paul, peter"

❌Using command line parameter fails (event not with just a single name handed over:

schematics .:hello --debug=false --name="paul"
Error: Schematic input does not validate against the Schema: {"name":"paul"}
Errors:

  Data path ".name" should be array.
    at MapSubscriber.registry.compile.pipe.operators_1.map.result [as project] (/usr/local/lib/node_modules/@angular-devkit/schematics-cli/node_modules/@angular-devkit/schematics/tools/schema-option-transform.js:31:27)
    at MapSubscriber._next (/usr/local/lib/node_modules/@angular-devkit/schematics-cli/node_modules/rxjs/internal/operators/map.js:49:35)
    at MapSubscriber.Subscriber.next (/usr/local/lib/node_modules/@angular-devkit/schematics-cli/node_modules/rxjs/internal/Subscriber.js:66:18)
    at ThrowIfEmptySubscriber._next (/usr/local/lib/node_modules/@angular-devkit/schematics-cli/node_modules/rxjs/internal/operators/throwIfEmpty.js:44:26)
    at ThrowIfEmptySubscriber.Subscriber.next (/usr/local/lib/node_modules/@angular-devkit/schematics-cli/node_modules/rxjs/internal/Subscriber.js:66:18)
    at TakeSubscriber._next (/usr/local/lib/node_modules/@angular-devkit/schematics-cli/node_modules/rxjs/internal/operators/take.js:54:30)
    at TakeSubscriber.Subscriber.next (/usr/local/lib/node_modules/@angular-devkit/schematics-cli/node_modules/rxjs/internal/Subscriber.js:66:18)
    at MergeMapSubscriber.notifyNext (/usr/local/lib/node_modules/@angular-devkit/schematics-cli/node_modules/rxjs/internal/operators/mergeMap.js:92:26)
    at InnerSubscriber._next (/usr/local/lib/node_modules/@angular-devkit/schematics-cli/node_modules/rxjs/internal/InnerSubscriber.js:28:21)
    at InnerSubscriber.Subscriber.next (/usr/local/lib/node_modules/@angular-devkit/schematics-cli/node_modules/rxjs/internal/Subscriber.js:66:18)

🔥 Exception or Error

Error: Schematic input does not validate against the Schema: {"name":"paul"}
Errors:

  Data path ".name" should be array.
    at MapSubscriber.registry.compile.pipe.operators_1.map.result [as project] (/usr/local/lib/node_modules/@angular-devkit/schematics-cli/node_modules/@angular-devkit/schematics/tools/schema-option-transform.js:31:27)
    at MapSubscriber._next (/usr/local/lib/node_modules/@angular-devkit/schematics-cli/node_modules/rxjs/internal/operators/map.js:49:35)
    at MapSubscriber.Subscriber.next (/usr/local/lib/node_modules/@angular-devkit/schematics-cli/node_modules/rxjs/internal/Subscriber.js:66:18)
    at ThrowIfEmptySubscriber._next (/usr/local/lib/node_modules/@angular-devkit/schematics-cli/node_modules/rxjs/internal/operators/throwIfEmpty.js:44:26)
    at ThrowIfEmptySubscriber.Subscriber.next (/usr/local/lib/node_modules/@angular-devkit/schematics-cli/node_modules/rxjs/internal/Subscriber.js:66:18)
    at TakeSubscriber._next (/usr/local/lib/node_modules/@angular-devkit/schematics-cli/node_modules/rxjs/internal/operators/take.js:54:30)
    at TakeSubscriber.Subscriber.next (/usr/local/lib/node_modules/@angular-devkit/schematics-cli/node_modules/rxjs/internal/Subscriber.js:66:18)
    at MergeMapSubscriber.notifyNext (/usr/local/lib/node_modules/@angular-devkit/schematics-cli/node_modules/rxjs/internal/operators/mergeMap.js:92:26)
    at InnerSubscriber._next (/usr/local/lib/node_modules/@angular-devkit/schematics-cli/node_modules/rxjs/internal/InnerSubscriber.js:28:21)
    at InnerSubscriber.Subscriber.next (/usr/local/lib/node_modules/@angular-devkit/schematics-cli/node_modules/rxjs/internal/Subscriber.js:66:18)

🌍 Your Environment

ng version
     _                      _                 ____ _     ___
    / \   _ __   __ _ _   _| | __ _ _ __     / ___| |   |_ _|
   / △ \ | '_ \ / _` | | | | |/ _` | '__|   | |   | |    | |
  / ___ \| | | | (_| | |_| | | (_| | |      | |___| |___ | |
 /_/   \_\_| |_|\__, |\__,_|_|\__,_|_|       \____|_____|___|
                |___/
    

Angular CLI: 9.0.0-rc.4
Node: 10.14.0
OS: darwin x64

Angular: undefined
... 
Ivy Workspace: Yes

Package                      Version
------------------------------------------------------
@angular-devkit/architect    0.900.0-rc.4 (cli-only)
@angular-devkit/core         9.0.0-rc.4
@angular-devkit/schematics   9.0.0-rc.4
@schematics/angular          9.0.0-rc.4 (cli-only)
@schematics/update           0.900.0-rc.4 (cli-only)
rxjs                         6.5.3
typescript                   3.6.4