Add expectAssignable expectation - fixes #39

readme.md

@@ -144,6 +144,10 @@ These options will be overridden if a `tsconfig.json` file is found in your proj
 
 Check that `value` is identical to type `T`.
 
+### expectAssignable<T>(value)
+
+Check that `value` is assignable to type `T`.
+
 ### expectError(function)
 
 Check if the function call has argument type errors.

source/lib/assertions/assert.ts

@@ -8,6 +8,16 @@ export const expectType = <T>(value: T) => {		// tslint:disable-line:no-unused
 	// Do nothing, the TypeScript compiler handles this for us
 };
 
+/**
+ * Check that `value` is assignable to type `T`.
+ *
+ * @param value - Value that should be assignable to type `T`.
+ */
+// @ts-ignore
+export const expectAssignable = <T>(value: T) => {		// tslint:disable-line:no-unused
+	// Do nothing, the TypeScript compiler handles this for us
+};
+
 /**
  * Assert the value to throw an argument error.
  *

source/test/assignability.ts

@@ -0,0 +1,12 @@
+import * as path from 'path';
+import test from 'ava';
+import {verify} from './fixtures/utils';
+import tsd from '..';
+
+test('assignable', async t => {
+	const diagnostics = await tsd({cwd: path.join(__dirname, 'fixtures/assignability/assignable')});
+
+	verify(t, diagnostics, [
+		[8, 26, 'error', 'Argument of type \'string\' is not assignable to parameter of type \'boolean\'.']
+	]);
+});

source/test/fixtures/assignability/assignable/index.d.ts

@@ -0,0 +1,6 @@
+declare const concat: {
+	(foo: string, bar: string): string;
+	(foo: number, bar: number): number;
+};
+
+export default concat;

source/test/fixtures/assignability/assignable/index.js

@@ -0,0 +1,3 @@
+module.exports.default = (foo, bar) => {
+	return foo + bar;
+};

source/test/fixtures/assignability/assignable/index.test-d.ts

@@ -0,0 +1,8 @@
+import {expectAssignable} from '../../../..';
+import concat from '.';
+
+expectAssignable<string | number>(concat('foo', 'bar'));
+expectAssignable<string | number>(concat(1, 2));
+expectAssignable<any>(concat(1, 2));
+
+expectAssignable<boolean>(concat('unicorn', 'rainbow'));

source/test/fixtures/assignability/assignable/package.json

@@ -0,0 +1,6 @@
+{
+	"name": "foo",
+	"dependencies": {
+		"rxjs": "^6.5.3"
+	}
+}