Port microsoft/TypeScript PR #62391: Switch libReplacement to false by default #1852
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…pectations Update the TypeScript submodule to include PR #62391 which switches libReplacement default from true to false. The Go code already had the correct implementation, so no code changes were needed. Updated test expectations for new transformer files added to the TypeScript codebase and accepted baseline changes.
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[WIP] Port changes from microsoft/TypeScript#62391
Port microsoft/TypeScript PR #62391: Switch libReplacement to false by default
Oct 9, 2025
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RyanCavanaugh
October 9, 2025 18:54
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This PR ports microsoft/TypeScript#62391, which changes the
libReplacementcompiler option default fromtruetofalse, making library replacement an opt-in feature.Background
The
libReplacementoption controls whether TypeScript attempts to resolve library files (likelib.es2015.d.ts) from@typescript/lib-*npm packages instead of using the built-in library files. PR #62391 in TypeScript changed this behavior from opt-out to opt-in.Changes Made
The Go codebase already had the correct implementation of this feature:
internal/tsoptions/declscompiler.go: Already hasDefaultValueDescription: falseinternal/compiler/fileloader.go: Already usesLibReplacement.IsTrue()which only enables lib replacement when explicitly set totrueTherefore, the main changes in this PR are:
1ee9e0d9ato1112daeadto include PR #62391 and bring in the latest changesinternal/tsoptions/tsconfigparsing_test.goto include new transformer files (es2015.tsandgenerators.ts) that were added to the TypeScript sourceBehavior
With this change, the behavior is now:
libReplacementisundefined(not specified): Uses default TypeScript libs ✓libReplacementis explicitlyfalse: Uses default TypeScript libs ✓libReplacementis explicitlytrue: Attempts to resolve replacement libs from@typescript/lib-*packagesThis aligns the Go implementation with the updated TypeScript behavior where library replacement is opt-in rather than default.
Testing
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