Revive the codegen typecheck harness

codegen-test/.gitignore

@@ -0,0 +1 @@
+types.ts

codegen-test/fixtures/geojson.ts

@@ -0,0 +1,121 @@
+// Type-level test for the generated GeoJSON codegen type.
+//
+// Typechecking `types.ts` on its own only proves the emitted type is *valid*
+// TypeScript. The GeoJSON precedence bug produced a valid-but-wrong type
+// (`Position | Position[]` without parentheses before `[]`), so `tsc` accepted
+// it. Assigning real values is what exercises the type: an arrayed geometry's
+// coordinates stop being assignable the moment the union isn't grouped.
+//
+// `input` and `output` are the same union for this scalar; testing one covers both.
+import type { Scalars } from '../types';
+
+type GeoJSON = Scalars['GeoJSON']['input'];
+
+// Valid — every geometry from the scalar's own validExamples must be assignable.
+const point: GeoJSON = {
+  type: 'Point',
+  coordinates: [100.0, 0.0],
+};
+
+const multiPoint: GeoJSON = {
+  type: 'MultiPoint',
+  coordinates: [
+    [100.0, 0.0],
+    [101.0, 1.0],
+  ],
+};
+
+const lineString: GeoJSON = {
+  type: 'LineString',
+  coordinates: [
+    [100.0, 0.0],
+    [101.0, 1.0],
+  ],
+};
+
+const multiLineString: GeoJSON = {
+  type: 'MultiLineString',
+  coordinates: [
+    [
+      [100.0, 0.0],
+      [101.0, 1.0],
+    ],
+    [
+      [102.0, 2.0],
+      [103.0, 3.0],
+    ],
+  ],
+};
+
+const polygon: GeoJSON = {
+  type: 'Polygon',
+  coordinates: [
+    [
+      [100.0, 0.0],
+      [101.0, 0.0],
+      [101.0, 1.0],
+      [100.0, 1.0],
+      [100.0, 0.0],
+    ],
+  ],
+};
+
+const multiPolygon: GeoJSON = {
+  type: 'MultiPolygon',
+  coordinates: [
+    [
+      [
+        [102.0, 2.0],
+        [103.0, 2.0],
+        [103.0, 3.0],
+        [102.0, 3.0],
+        [102.0, 2.0],
+      ],
+    ],
+  ],
+};
+
+const geometryCollection: GeoJSON = {
+  type: 'GeometryCollection',
+  geometries: [{ type: 'Point', coordinates: [100.0, 0.0] }],
+};
+
+const feature: GeoJSON = {
+  type: 'Feature',
+  geometry: { type: 'Point', coordinates: [100.0, 0.0] },
+  properties: { name: 'Test Point' },
+};
+
+const featureCollection: GeoJSON = {
+  type: 'FeatureCollection',
+  features: [
+    {
+      type: 'Feature',
+      geometry: { type: 'Point', coordinates: [100.0, 0.0] },
+      properties: { name: 'Test Point' },
+    },
+  ],
+};
+
+// Invalid — a single Position is not an array of Positions. The unparenthesized
+// bug wrongly accepts this; then the directive is unused and tsc fails, catching
+// the bug from the "accepts too much" side and guarding against widening to `any`.
+const barePosition: GeoJSON = {
+  type: 'MultiPoint',
+  // @ts-expect-error coordinates must be an array of Positions, not one Position
+  coordinates: [100.0, 0.0],
+};
+
+// Reference every binding so unused-local checks stay quiet.
+void [
+  point,
+  multiPoint,
+  lineString,
+  multiLineString,
+  polygon,
+  multiPolygon,
+  geometryCollection,
+  feature,
+  featureCollection,
+  barePosition,
+];

codegen-test/package.json

@@ -1,7 +1,15 @@
 {
   "name": "codegen-scalars-test",
+  "scripts": {
+    "codegen": "graphql-codegen --config codegen.yml",
+    "test": "yarn codegen && tsc --noEmit -p tsconfig.json"
+  },
   "dependencies": {
     "@graphql-codegen/cli": "6.2.1",
     "@graphql-codegen/typescript": "5.0.9"
+  },
+  "devDependencies": {
+    "@types/node": "^22.0.0",
+    "typescript": "^5.5.0"
   }
 }

codegen-test/tsconfig.json

@@ -1,8 +1,15 @@
 {
   "compilerOptions": {
+    "noEmit": true,
+    "skipLibCheck": true,
+    "strict": false,
+    "moduleResolution": "node",
+    "lib": ["ESNext", "DOM"],
+    "types": ["node"],
+    "baseUrl": ".",
     "paths": {
       "graphql-scalars": ["../src/index.ts"]
     }
   },
-  "files": ["./types.ts"]
+  "include": ["./types.ts", "./fixtures/**/*.ts"]
 }