[Feature] Support merging MappedCoverage with FileCoverage from the same files

[mgfalzon]

Various test runners (Jest, Vitest, Playwright) support a sharding option with --shard. When test runners run in this mode coverage is generated differently in each shard.

Example:
If A.ts is a dependency of the tests in --shard=1/2 and not a dependency of the tests in --shard=2/2 then the first shard's coverage report will reference a MappedCoverage instance of A.ts but the second shard will reference a FileCoverage instance.

Attempting to merge these files using istanbuljs will lead to inaccurate coverage. Presumably due to the mismatch in line numbers.

Workaround / Potential Solution
To work around this issue we can apply patches to istanbul-lib-coverage and istanbul-lib-source-maps which adds a new isMapped property denoting whether or not a file is an instance of MappedCoverage. We then prefer MappedCoverage over FileCoverage and prevent merges of MappedCoverage with FileCoverage for the same file.

istanbul-lib-coverage+3.2.2.patch
istanbul-lib-source-maps+4.0.0.patch

These patches only work when using the same TestRunner across merges! Solving for differing test-runners is a more complicated issue due to differences in transformation

Related Issues

• Merged results have a lower coverage rating nyc#1333
• Jest and Cypress's coverage merged results is wrong  nyc#1533
• Merging coverage files doesn't produce accurate cumulative result nyc#1302