fix(typescript): ensure LazyResult complies with Promise interface.

* Explicitly implement Promise<Result> in the type declaration
* Add mising [Symbol.toStringTag] property
* Add some type tests to ensure we can type `postcss(plugins).process` results

lib/lazy-result.d.ts

@@ -13,7 +13,7 @@ import Root from './root.js'
  * const lazy = postcss([autoprefixer]).process(css)
  * ```
  */
-export default class LazyResult {
+export default class LazyResult implements Promise<Result> {
   /**
    * Processes input CSS through synchronous and asynchronous plugins
    * and calls `onFulfilled` with a Result instance. If a plugin throws
@@ -66,6 +66,12 @@ export default class LazyResult {
    */
   constructor (processor: Processor, css: string, opts: ResultOptions)
 
+  /**
+   * Returns the default string description of an object.
+   * Required to implement the Promise interface.
+   */
+  get [Symbol.toStringTag] (): string
+
   /**
    * Returns a `Processor` instance, which will be used
    * for CSS transformations.

test/types.ts

@@ -1,4 +1,4 @@
-import { PluginCreator } from '../lib/postcss.js'
+import postcss, { Result, PluginCreator, SourceMap } from '../lib/postcss.js'
 
 const plugin: PluginCreator<string> = prop => {
   return {
@@ -14,4 +14,19 @@ const plugin: PluginCreator<string> = prop => {
 
 plugin.postcss = true
 
+interface StyleCompileResults {
+  code: string
+  map: SourceMap | undefined
+}
+
+const processResult: Promise<Result> | Result = postcss([
+  plugin
+]).process('h1{color: black;}', { from: undefined })
+const processed:
+  | StyleCompileResults
+  | Promise<StyleCompileResults> = processResult.then(result => ({
+  code: result.css,
+  map: result.map
+}))
+
 export default plugin