chore: type check tests via lint:tsc

.gitignore

@@ -38,6 +38,7 @@ node_modules/
 
 # TypeScript cache
 *.tsbuildinfo
+**/.tsc-lint-cache/
 
 # Optional npm cache directory
 .npm

eslint.config.mjs

@@ -11,7 +11,13 @@ const tsconfigRootDir = path.dirname(fileURLToPath(import.meta.url));
 const config = createConfig([
   ...base,
   {
-    ignores: ['coverage/**', 'dist/**', 'docs/**', '.yarn/**'],
+    ignores: [
+      '**/.tsc-lint-cache',
+      'coverage/**',
+      'dist/**',
+      'docs/**',
+      '.yarn/**',
+    ],
   },
   {
     linterOptions: {

package.json

@@ -37,14 +37,15 @@
     "build:clean": "yarn build:only-clean && yarn build",
     "build:docs": "typedoc",
     "build:only-clean": "rimraf ./dist ./tsconfig.build.tsbuildinfo",
-    "lint": "yarn lint:eslint && yarn lint:constraints && yarn lint:misc --check && yarn lint:dependencies && yarn lint:changelog",
+    "lint": "yarn lint:tsc && yarn lint:eslint && yarn lint:constraints && yarn lint:misc --check && yarn lint:dependencies && yarn lint:changelog",
     "lint:changelog": "auto-changelog validate --prettier",
     "lint:constraints": "yarn constraints",
     "lint:dependencies": "knip --config knip.config.mts --dependencies && yarn dedupe --check",
     "lint:dependencies:fix": "knip --config knip.config.mts --dependencies && yarn dedupe",
     "lint:eslint": "eslint . --cache",
     "lint:fix": "yarn lint:eslint --fix && yarn lint:constraints --fix && yarn lint:misc --write && yarn lint:dependencies:fix && yarn lint:changelog",
     "lint:misc": "oxfmt --ignore-path .gitignore",
+    "lint:tsc": "tsc --project tsconfig.lint.json",
     "prepack": "./scripts/prepack.sh",
     "test": "yarn test:source && yarn test:types",
     "test:source": "jest && jest-it-up --config jest.config.cjs",

tsconfig.lint.json

@@ -0,0 +1,37 @@
+{
+  /**
+   * Type checks everything the build does not: tests, fixtures and the config
+   * files at the root. `tsconfig.build.json` only covers `src` minus tests, and
+   * ts-jest runs in transpile-only mode because `isolatedModules` is set, so
+   * without this nothing checks the types in a test file.
+   */
+  "extends": "./tsconfig.json",
+  "compilerOptions": {
+    /**
+     * Dependencies ship declarations that do not satisfy the strict flags used
+     * here, and their errors are not ours to fix.
+     */
+    "skipLibCheck": true,
+    /**
+     * Emit into a hidden, git ignored cache directory, matching how `lint:tsc`
+     * works in core. Emitting is what makes `tsBuildInfoFile` meaningful, so
+     * repeat runs only check what changed. Only declarations are written, since
+     * the JavaScript would be thrown away. `tsconfig.json` sets `noEmit` for the
+     * editor and `tsconfig.build.json` owns the real build, so both are
+     * overridden here rather than the other way round.
+     */
+    "noEmit": false,
+    "declaration": true,
+    "emitDeclarationOnly": true,
+    "incremental": true,
+    "outDir": "./.tsc-lint-cache",
+    "tsBuildInfoFile": "./.tsc-lint-cache/tsconfig.tsbuildinfo"
+  },
+  /**
+   * `*.test-d.ts` files belong to `tsd`, which type checks them itself via
+   * `yarn test:types`. Pulling them in here would drag `tsd`'s own types into
+   * the program, and those reference `lib="esnext"`, which widens built in
+   * types like `ReadonlySet` well beyond the `lib` this package targets.
+   */
+  "exclude": ["./dist/**/*", "./src/**/*.test-d.ts"]
+}