Coverage report is wrong (lower) when code is executed by many separate test files

[ipsips]

🐛 Bug Report

Since upgrading to jest 27 (i have 27.0.6 atm) the coverage is not reported correctly. My project is massive in which some utility modules are executed in lots of unit tests while these utilities do not have unit tests of their own. Prior to jest 27 this wasn't a problem as other test exercised these utilities fully and thus they were covered 100%. Now it seems that some of that coverage gets lost

To Reproduce

Clone https://github.com/ipsips/jest-coverage-problem and issue npm test.

Expected behavior

Expecting 100% coverage on get1.ts but I consistently get less:

----------|---------|----------|---------|---------|-------------------
File      | % Stmts | % Branch | % Funcs | % Lines | Uncovered Line #s 
----------|---------|----------|---------|---------|-------------------
All files |   81.82 |    66.67 |     100 |   81.82 |                   
 get1.ts  |   66.67 |       50 |     100 |   66.67 | 5-6               
 get2.ts  |     100 |      100 |     100 |     100 |                   
...
 get99.ts |     100 |      100 |     100 |     100 |                   
----------|---------|----------|---------|---------|-------------------

If you delete get3.ts through get100.ts and leave only get1.ts and get2.ts (with their spec files ofc) then coverage is 100% as expected.

Link to repl or repo (highly encouraged)

https://github.com/ipsips/jest-coverage-problem

envinfo

this problem also occurs for others in my team with various op systems

[ipsips]

possible duplicate: #11647

[ipsips]

if any moderator can remove "Need Repro" tag..

[ipsips]

bumping this for this is a serious blocker for us and no clue as to where to start looking for the bug :/

[ILokalin]

I found the same problem when working with branches. A similar functional component that checks the state before rendering is where the coverage randomly shows uncovered lines. However, the component has its own test file and a 2 snapshot match

3 of 4 runs result:

4x    if (!data.length) return null;
2x 
2x    return (
2x        <Wrapper>
2x            <SomeComponent data={data} />
2x        </Wrapper>
2x    );

1 of 4 runs result (all tests passed without errors):

2x    if (!data.length) return null;
 -
 -    return (
 -        <Wrapper>
 -            <SomeComponent data={data} />
 -        </Wrapper>
 -    );

My other components with missing coverage also had branches.
I tried --no-cache key - the coverage did not change

"jest": "26.6.3"

[abendi]

Changing coverageProvider from v8 (experimental) to babel (default) seems to fix the issue for author's repro example.

[sisiea]

Changing coverageProvider from v8 (experimental) to babel (default) seems to fix the issue for author's repro example.

Use "babel" coverageProvider work fine for me. But can anybody explain why "v8" coverageProvider did not work ?

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[github-actions]

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