Typecheck with the native TypeScript compiler (TS 7 / tsgo): tsc is now the forge's top CPU consumer #1870

The forge's check now typechecks with the native TypeScript compiler: typescript is 7.0.2, the Go port, published as plain typescript on npm. @typescript/native-preview is the older preview line, last published in July. It's a dev dependency only; dependencies is still empty.

Before/after on this repo (same tree, same machine, plain --noEmit, three runs each):

That's about 10x on wall time and 3–4x on CPU.

What changed

• forge.json: both check commands run GOMEMLIMIT=2GiB ./node_modules/typescript/bin/tsc --noEmit --incremental …. The compiler is now a Go binary, so the 2 GB bound is GOMEMLIMIT instead of node's --max-old-space-size.

- An old 5.x .tsbuildinfo in .forge-cache/ is safe: 7.0.2 ignores it and writes its own. The first check after this lands runs cold (~3 s).

• TypeScript 5.9 kept under an alias: scripts/hollow.ts walks the AST through 5.x's JS API, and 7.0 has no stable one. So 5.9 stays as the dev dependency typescript-api (npm:typescript@^5.9.0), imported only there.

- On fresh installs node_modules/.bin/tsc links to 7.x. A new test in test/shards.test.ts fails if the two packages are swapped.

• Builder brief: the text is unchanged. bun x tsc --noEmit now runs 7.0.2 here, and in any repo with no TypeScript pin, because 7.0.2 is npm's latest. A repo that pins 5.x still gets 5.x, which matches what that repo's own check runs. I chose not to force typescript@7 there, because TypeScript 7 drops options that 5.x configs may still use.

Where the two compilers disagree

• TS 7 reports something tsc 5.9 doesn't (false positive): a raw U+FFFD as a range bound in a u-flag regex class gives TS1516 in 7.0.2. My guess is it's mistaken for Go's UTF-8 error rune.

- This was the only error on this tree: src/web/og/card.ts's XML-character regex. - I rewrote its bounds as \uD7FF\uE000-\uFFFD. It's the same regex, both compilers pass it, and it no longer contains an invisible U+E000. - A new test in test/og-card.test.ts pins both ends of each XML 1.0 character range. - Since rewriting the regex removed the disagreement, nothing needs tsc 5 kept for that case.

• TS 7 reports something tsc 5.9 doesn't (stricter, kept): a side-effect import that doesn't resolve is TS2882, because noUncheckedSideEffectImports is on by default in 6/7.
• Seeded-error spot check: I added a scratch file with an error for each strict flag in tsconfig.json. Both compilers reported the same seven diagnostics; the only extra was the TS2882 above.

Not fixed here: while writing the regex test I found that Bun's regex engine (not V8) lets a lone high surrogate followed by U+FFFE through clean()'s global replace. It's a runtime bug, so I left it alone.

To try it: bun install && bun x tsc --noEmit prints Version 7.0.2 with bun x tsc --version. The before numbers come from node node_modules/typescript-api/lib/tsc.js --noEmit. The covering and cross test files from CLAUDE.md all pass (322 tests, 8 files), and bun scripts/gate.ts passes. The exact forge.json typecheck step passes when run through sh.

🤖 Generated with Claude Code

Closes #1864