fix(lib-coverage): merge ranges that differ between transforms

[stevez]

• Fixes coverage-v8: Duplicate statements when merging coverage from multiple projects with different environments vitest#9366
• Supersedes fix: use lenient matching for coverage merge when end.column differs istanbuljs/istanbuljs#838 and fix(coverage): apply patch from istanbuljs/istanbuljs#838 vitest#9480

Problem

When the same file is covered by multiple transforms, FileCoverage#merge() duplicates statements, functions and branches. For example, Vitest projects with jsdom + browser, or a plain project + one using SWC. Items are matched by their exact start|end location, but transforms can disagree on columns:

• Statements/branches: the end column differs. SSR transforms produce end.column: null, browser transforms resolve it (1:22->7:null vs 1:22->7:10).
• Functions: the body range (loc) differs in start column (1:42 vs 1:40 for export function sum(a: number, b: number) {), while the declaration range (decl) is identical. This is why the earlier end-column-only approach in istanbuljs#838 still duplicated functions (review).

Fix

mergeProp keeps exact matching and adds an optional fallback key for items that have no exact match:

• statements and branches: location ignoring the end column
• functions: the decl range

Fallback matches are only applied when they're one-to-one. If multiple items share a fallback key, nothing is collapsed and the existing nearest-container logic applies, so no hits are dropped. When matched items differ, the one with a resolved end column is kept.

Tests

New unit tests are built from hand-made coverage objects using locations captured from real Vitest runs, so there's no SWC or third-party plugin dependency:

• statements with null vs resolved end columns
• functions whose body start column differs (the math.ts case from the Vitest review)
• branches with different end columns
• ambiguous fallback matches are not collapsed
• identical null-end ranges still merge (the case that crashed istanbuljs#838 earlier)
• functions without decl don't throw

The 3 merge tests fail without the fix. The full suite passes after pnpm build.

Verified end-to-end

I swapped the built @vitest/istanbul-lib-coverage into Vitest 5.0.3 projects:

• https://github.com/stevez/vitest-coverage-merge-bug: constants.ts goes from 6 statements to 3
• A Next.js app with jsdom + browser projects: 2 of 8 shared files had inflated counts (constants.ts 29 → 15, validators.ts 27 → 19). All 8 now match the single-project counts.

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Note

This PR was written by Claude (AI agent, Claude Code), working with @stevez. @stevez reviewed the approach and the verification results before it was opened.

🤖 Generated with Claude Code