bump ts to 4.0, ensure no new type syntax

Author: Simon Holthausen

package-lock.json

@@ -83,7 +83,7 @@
     "pretest": "npm run build",
     "posttest": "agadoo internal/index.mjs",
     "prepublishOnly": "node check_publish_env.js && npm run lint && npm test",
-    "tsd": "tsc -p src/compiler --emitDeclarationOnly && tsc -p src/runtime --emitDeclarationOnly",
+    "tsd": "tsc -p src/compiler --emitDeclarationOnly && tsc -p src/runtime --emitDeclarationOnly && node ./post-typegen.js",
     "lint": "eslint \"{src,test}/**/*.{ts,js}\""
   },
   "repository": {
@@ -140,7 +140,7 @@
     "sourcemap-codec": "^1.4.8",
     "tiny-glob": "^0.2.6",
     "tslib": "^2.0.3",
-    "typescript": "^3.7.5"
+    "typescript": "~4.0.0"
   },
   "nyc": {
     "include": [

package.json

@@ -0,0 +1,14 @@
+// Svelte 3 types are generated using TS 3.7 . Using newer type syntax is therefore technically a breaking change.
+// `export type` / `import type` was introduced later, and needed now in one of the files.
+// Replace `export type` with `export` in the `d.ts` file as it doesn't make a difference in `d.ts` files.
+// This keeps backwards-compatibility
+
+const fs = require('fs');
+
+const path = 'types/runtime/index.d.ts';
+const content = fs.readFileSync(path, 'utf8');
+const replaced = content.replace('export type', 'export');
+if (content === replaced) {
+    throw new Error('types/runtime/index.d.ts changed. Update post-typegen.js')
+}
+fs.writeFileSync(path, replaced);