fix: tsconfig extends supporting same extensions (inc .jsonc) for relative and package imports (bug #43121)

[domdomegg]

Fixes #43121

This PR adds support for .jsonc file extensions when resolving tsconfig extends paths through Node.js module resolution. Previously, TypeScript only supported .jsonc files for relative paths (e.g., "extends": "./config.jsonc") but failed when trying to resolve package-based extends (e.g., "extends": "pkg/config.jsonc").

Problem

When using "extends": "pkg/foo.jsonc" in a tsconfig.json, TypeScript would attempt to resolve node_modules/pkg/foo.jsonc.json and fail, even though the same .jsonc file could be successfully loaded using relative paths like "extends": "./foo.jsonc".

This inconsistency forced developers to either:

• Use .json extensions and deal with linter warnings about comments
• Rely on special-case handling by dev tools for tsconfig*.json files
• Avoid package-based config sharing with .jsonc files

Changes

Core Resolution Changes

• src/compiler/moduleNameResolver.ts:

  • Renamed nodeNextJsonConfigResolver to nodeNextTsconfigResolver
  • Changed config lookup to use Extensions.Any instead of Extensions.Json. I was thinking about adding a Jsonc Extensions value, but decided against this so the behaviour of relative tsconfig and package tsconfig imports would be the same (any extensions are valid for relative imports)

• src/compiler/commandLineParser.ts:

  • Updated import and function call to use the renamed resolver

Testing

Added test cases before starting, to ensure test went from failing to passing:

• tests/cases/compiler/tsconfigExtendsJsoncPackage.ts: Tests package-based .jsonc extends
• tests/cases/compiler/tsconfigExtendsJsoncRelative.ts: Ensures relative .jsonc extends still work (regression test)

Both test cases verify that:

• The .jsonc config files are properly resolved and loaded
• Compiler options from the extended config are applied correctly
• Source maps are generated as expected when inlineSourceMap: true is set

Backward Compatibility

This change is fully backward compatible:

• Existing .json config files continue to work exactly as before
• Relative .jsonc paths that already worked remain unchanged

[domdomegg]

@microsoft-github-policy-service agree

[fluiderson]

Does this fully fix the issue with TypeScript dismissing JSONC config files? #60261 was marked as duplicate of #43121, it describes that TS also can't process JSONC in path for references. Could you include tests for this, too?

[typescript-bot]

With 6.0 out as the final release vehicle for this codebase, we're closing all PRs that don't fit the merge criteria for post-6.0 patches. If you think this was a mistake and this PR fits the post-6.0 patch criteria, please post to the 6.0 iteration issue with details (specifically, which PR and which patch criteria it satisfies).

Next steps for PRs:

• For crash bugfixes or language service improvements, PRs are currently accepted at the typescript-go repo
• Changes to type system behavior should wait until after 7.0, at which point mainline TypeScript development will resume in this repository with the Go codebase
• Library file updates (lib.d.ts etc) continue to live in this repo or the DOM Generator repo as appropriate