Schematics: Unit testing a rule that uses mergeWith/apply fails due to missing context

Start with the linked minimal reproduction and the unit test using callRule, mergeWith, and apply. Trace the reported failure through file-system-engine-host-base.js and the schematics url.js entry point, paying attention to the omitted partialContext. Done should establish the required context and either make the test pass or clearly report the missing context.

Addendum

In creating the test case I've realised that I'm not passing a partialContext to callRule, so it is definitely missing context, however, it is not at all clear what context is necessary for mergeWith to succeed and the error does not indicate that context is actually missing.

Happy for this to be marked as a question instead. Guidance on how to provide the context and what context to provide would be amazingly useful.

🐞 Bug report

Description

Using callRule to unit test rules within a schematic fails when it comes to rules which return mergeWith

🔬 Minimal Reproduction

See: https://github.com/jdpearce/schematics-apply-problem

Given a rule which applies template files to the tree, this unit test will fail with TypeError: Cannot read property 'path' of undefined:

describe('applyTemplateFiles Rule', () => {
  let tree: UnitTestTree;

  beforeEach(async () => {
    tree = new UnitTestTree(Tree.empty());
  });

  it('should apply the files to the tree', async () => {
    const schema = {};

    tree = (await callRule(applyTemplateFiles(schema), tree)) as UnitTestTree;

    expect(tree.files).toEqual(jasmine.arrayContaining(['/libs/destination/test.txt']));
  });
});

🔥 Exception or Error

Failures:
1) applyTemplateFiles Rule should apply the files to the tree
  Message:
    TypeError: Cannot read property 'path' of undefined
  Stack:
        at <Jasmine>
        at ./apply-problem/node_modules/@angular-devkit/schematics/tools/file-system-engine-host-base.js:216:96
        at ./tmp/apply-problem/node_modules/@angular-devkit/schematics/src/rules/url.js:13:73
        at Object.callSource (./tmp/apply-problem/node_modules/@angular-devkit/schematics/src/rules/call.js:55:20)
        at ./tmp/apply-problem/node_modules/@angular-devkit/schematics/src/rules/base.js:45:60
        at Object.callSource (./tmp/apply-problem/node_modules/@angular-devkit/schematics/src/rules/call.js:55:20)
        at ./tmp/apply-problem/node_modules/@angular-devkit/schematics/src/rules/base.js:53:23
        at MergeMapSubscriber.project (./tmp/apply-problem/node_modules/@angular-devkit/schematics/src/rules/call.js:74:24)
        at MergeMapSubscriber._tryNext (./tmp/apply-problem/node_modules/rxjs/internal/operators/mergeMap.js:69:27)
        at MergeMapSubscriber._next (./tmp/apply-problem/node_modules/rxjs/internal/operators/mergeMap.js:59:18)
        at MergeMapSubscriber.Subscriber.next (./tmp/apply-problem/node_modules/rxjs/internal/Subscriber.js:66:18)

2 specs, 1 failure

🌍 Your Environment

"@angular-devkit/core": "^9.0.6",
"@angular-devkit/schematics": "^9.0.6",
"@types/jasmine": "^3.3.9",
"@types/node": "^8.0.31",
"jasmine": "^3.3.1",
"typescript": "~3.5.3"