[istanbul-lib-coverage] After istanbul-lib-coverage merge, unexpected branch not covered at trailing char {

[loynoir]

Brief

After istanbul-lib-coverage merge, unexpected branch not covered at trailing char {

Description

Run reproduce.mjs with env RUNTIME=node and RUNTIME=browser.

Merge coverage-final.json from different runtime.

Expect 100% coverage.

Actually, branch not covered at trailing char {

---------------|---------|----------|---------|---------|-------------------
File           | % Stmts | % Branch | % Funcs | % Lines | Uncovered Line #s 
---------------|---------|----------|---------|---------|-------------------
All files      |     100 |    33.33 |     100 |     100 |                   
 reproduce.mjs |     100 |    33.33 |     100 |     100 | 4-8               
---------------|---------|----------|---------|---------|-------------------

reproduce.mjs:1:const IS_NODE = process.env['RUNTIME'].includes('node')
reproduce.mjs:2:const IS_BROWSER = process.env['RUNTIME'].includes('browser')
reproduce.mjs:3:
reproduce.mjs:4:if (IS_NODE) {
reproduce.mjs:5:  console.log('node')
reproduce.mjs:6:}
reproduce.mjs:7:
reproduce.mjs:8:if (IS_BROWSER) {
reproduce.mjs:9:  console.log('browser')
reproduce.mjs:10:}
reproduce.mjs:11:
reproduce.mjs:12:export { }

Reproduce Repo

https://github.com/loynoir/reproduce-istanbul-lib-coverage-737

Related

https://github.com/ljharb/istanbul-merge/blob/354e660ee6c2a37806d521f90cfaace5eb2a3f4e/bin/istanbul-merge#L36

https://playwright.dev/docs/api/class-coverage

[ljharb]

I can reproduce this issue using istanbul-merge, which is a thin wrapper around istanbul.createCoverageMap and .merge, so I think it's a bug with istanbul itself.