Branch coverage data is incorrect after upgrading to Angular 13

[acieroid]

🐞 Bug report

Command (mark with an x)

• new
• build
• serve
• test
• e2e
• generate
• add
• update
• lint
• extract-i18n
• run
• config
• help
• version
• doc

Is this a regression?

Yes, the previous version in which this bug was not present was: Angular 12

Description

The coverage produced when running ng test --code-coverage is incorrect, it is missing branches that were previously counted. This applies to if that do not have an else: these used to be counted as two branches, and they are now counted as one. This artificially increases branch coverage.

🔬 Minimal Reproduction

Starting from a minimal reproduction repository (ng new), make the following changes:

1. Add src/foo.ts. Note that this function contains two branches to cover: the if, and the (non-explicitly written) else.

export function foo(n: number) {
  if (n % 2 === 0) {
    console.log('hello');
  }
}

2. Add src/foo.spec.ts:

import { foo } from './foo';

describe('test', () => {
  it('test', () => {
    foo(2);
  });
});

3. Restrict the test suite to run only foo.spec.ts in src/test.ts:

const context = require.context('./', true, /foo\.spec\.ts$/);

4. Launch the tests with code coverage: npx ng test --code-coverage
5. The output reports full coverage, of only one branch:

=============================== Coverage summary ===============================
Statements   : 100% ( 2/2 )
Branches     : 100% ( 1/1 )
Functions    : 100% ( 1/1 )
Lines        : 100% ( 2/2 )

For comparison, downgrading to Angular 12 and to Typescript 4.3.5 results in the correct coverage report, which is:

=============================== Coverage summary ===============================
Statements   : 100% ( 5/5 )
Branches     : 50% ( 1/2 )
Functions    : 100% ( 1/1 )
Lines        : 100% ( 5/5 )

🌍 Your Environment

Everything comes from the default app when running ng new

Angular CLI: 13.2.2
Node: 16.13.2
Package Manager: npm 8.1.2
OS: linux x64

Angular: 13.2.1
... animations, common, compiler, compiler-cli, core, forms
... platform-browser, platform-browser-dynamic, router

Package                         Version
---------------------------------------------------------
@angular-devkit/architect       0.1302.2
@angular-devkit/build-angular   13.2.2
@angular-devkit/core            13.2.2
@angular-devkit/schematics      13.2.2
@angular/cli                    13.2.2
@schematics/angular             13.2.2
rxjs                            7.5.2
typescript                      4.5.5

[alan-agius4]

It turned out to be pretty complex to find the root cause. After spending most of yesterday afternoon and today's morning I was able to locate the source of the issue. It turns out that this is caused by this change in istanbul-lib-instrument which is causing incorrect branch coverage for implicit else.

I opened an issue in their repo with a minimal reproduction. I suggest that you subscribe to that issue.

I am going to close this issue, as is not actionable from our end since we don't control which version of istanbul-lib-instrument is used because this is a transitive dependency.

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