Snapshot test what compiler errors look like

package.json

@@ -33,11 +33,13 @@
   },
   "devDependencies": {
     "@types/jest": "29.0.0",
+    "@types/node": "^14.0.0",
     "eslint": "8.23.0",
     "eslint-plugin-mmkal": "0.0.1-2",
     "jest": "28.1.3",
     "np": "7.6.2",
     "ts-jest": "28.0.8",
+    "ts-morph": "16.0.0",
     "typescript": "4.8.2"
   }
 }

test/errors.test.ts

@@ -0,0 +1,41 @@
+import * as tsmorph from 'ts-morph'
+
+const tsErrors = (code: string) => {
+  const project = new tsmorph.Project()
+  project.addSourceFileAtPath('./src/index.ts')
+  project.createSourceFile('test.ts', `import {expectTypeOf} from './src'\n\n${code}`)
+  const diagnostics = project.getPreEmitDiagnostics()
+  return diagnostics.map(d => d.getMessageText())
+}
+
+test('toEqualTypeOf<...>() error message', async () => {
+  expect(tsErrors(`expectTypeOf({a: 1}).toEqualTypeOf<{a: string}>()`)).toMatchInlineSnapshot(`
+    Array [
+      "Expected 1 arguments, but got 0.",
+    ]
+  `)
+})
+
+test('toEqualTypeOf(...) error message', async () => {
+  expect(tsErrors(`expectTypeOf({a: 1}).toEqualTypeOf({a: 'one'})`)).toMatchInlineSnapshot(`
+    Array [
+      "Argument of type '{ a: string; }' is not assignable to parameter of type 'never'.",
+    ]
+  `)
+})
+
+test('toMatchTypeOf<...>() error message', async () => {
+  expect(tsErrors(`expectTypeOf({a: 1}).toMatchTypeOf<{a: string}>()`)).toMatchInlineSnapshot(`
+    Array [
+      "Expected 1 arguments, but got 0.",
+    ]
+  `)
+})
+
+test('toMatchTypeOf(...) error message', async () => {
+  expect(tsErrors(`expectTypeOf({a: 1}).toMatchTypeOf({a: 'one'})`)).toMatchInlineSnapshot(`
+    Array [
+      "Expected 2 arguments, but got 1.",
+    ]
+  `)
+})