fix: support TypeScript 7 declaration builds

[ljh12138164]

fix: support TypeScript 7 declaration builds

Closes #1408.

Summary

This PR adds TypeScript 7 support for declaration builds while preserving the
existing behavior for TypeScript 4.5 through 6.

TypeScript 7 projects can now use --dts and --experimental-dts by installing
the official @typescript/typescript6 compatibility package. Projects that do
not generate declarations do not need the compatibility package.

Problem

TypeScript 7 currently does not expose the legacy Compiler API used by tsup and
rollup-plugin-dts. With TypeScript 7.0.2 and dts: true, tsup 8.5.1 therefore
fails while initializing the declaration build. This is the failure reported in
#1408.

For example, APIs used by the existing declaration paths are unavailable from
the TypeScript 7 package:

• createProgram
• createCompilerHost
• parseJsonConfigFileContent
• sys

The JavaScript bundling path does not require these APIs and should continue to
work without any additional dependency.

Implementation

Upgrade rollup-plugin-dts

rollup-plugin-dts is upgraded from 6.1.1 to 6.5.1. This includes the
compatibility work from
rollup-plugin-dts PR #401:

1. Load the project's typescript package dynamically.
2. Use it directly when it exposes createProgram.
3. Fall back to @typescript/typescript6 when the installed TypeScript package
   does not expose the Compiler API.
4. Provide an actionable error when the compatibility package is required but
   missing.

Add a shared TypeScript API loader

Upgrading rollup-plugin-dts alone is not sufficient because tsup also uses the
Compiler API directly in its standard and experimental declaration paths.

This PR adds a shared loadTypeScript() helper that follows the same capability
detection strategy:

const typescript = require('typescript')

if (typeof typescript.createProgram === 'function') {
  return typescript
}

return require('@typescript/typescript6')

The loader is used by both:

• the Rollup-based --dts implementation;
• the TypeScript/API Extractor-based --experimental-dts implementation.

Loading remains lazy. The compatibility API is requested only after a
declaration build starts, so TypeScript 7 projects that only bundle JavaScript
do not need @typescript/typescript6.

Declare the compatibility package

@typescript/typescript6 is added as an optional peer dependency. TypeScript
4.5 through 6 continue using their installed Compiler API. TypeScript 7 users
that generate declarations install both packages:

npm install --save-dev typescript@7 @typescript/typescript6

The project still compiles with TypeScript 7. The TypeScript 6 compatibility API
is used only by declaration tooling. This follows Microsoft's documented
TypeScript 7 transition guidance.

Compatibility

Project setup	Declaration API used	Extra package required
TypeScript 4.5-6 with DTS	Installed typescript	No
TypeScript 7 with DTS	@typescript/typescript6	Yes
TypeScript 7 without DTS	None	No

Tests

Added coverage for:

• using the Compiler API from TypeScript 4.5 through 6;
• falling back to @typescript/typescript6 for TypeScript 7;
• reporting an actionable error when the fallback is missing;
• running a TypeScript 7 build without declarations and without the fallback;
• running an end-to-end TypeScript 7 declaration build with the fallback.

Full test result:

Test Files  11 passed (11)
Tests       100 passed (100)

References

• tsup issue #1408: TypeScript v7 support
• rollup-plugin-dts PR #401: TypeScript 7 compatibility
• Microsoft: Announcing TypeScript 7.0 RC

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Review the following changes in direct dependencies. Learn more about Socket for GitHub.

Diff	Package	Supply Chain Security	Vulnerability	Quality	Maintenance	License
	@​typescript/​typescript6@​6.0.2
	rollup-plugin-dts@​6.5.1

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