fix inferring the correct fn type

types/example-async.test-d.ts

@@ -0,0 +1,19 @@
+import { FastifyPluginAsync } from "fastify";
+
+type FastifyExampleAsync = FastifyPluginAsync<fastifyExampleAsync.FastifyExampleAsyncOptions>;
+
+declare namespace fastifyExampleAsync {
+
+  export interface FastifyExampleAsyncOptions {
+    foo?: 'bar'
+  }
+
+  export interface FastifyExampleAsyncPluginOptions extends FastifyExampleAsyncOptions {
+  }
+  export const fastifyExampleAsync: FastifyExampleAsync
+  export { fastifyExampleAsync as default }
+}
+
+declare function fastifyExampleAsync(...params: Parameters<FastifyExampleAsync>): ReturnType<FastifyExampleAsync>
+
+export default fastifyExampleAsync

types/example-callback.test-d.ts

@@ -0,0 +1,19 @@
+import { FastifyPluginCallback } from "fastify";
+
+type FastifyExampleCallback = FastifyPluginCallback<fastifyExampleCallback.FastifyExampleCallbackOptions>;
+
+declare namespace fastifyExampleCallback {
+
+  export interface FastifyExampleCallbackOptions {
+    foo?: 'bar'
+  }
+
+  export interface FastifyExampleCallbackPluginOptions extends FastifyExampleCallbackOptions {
+  }
+  export const fastifyExampleCallback: FastifyExampleCallback
+  export { fastifyExampleCallback as default }
+}
+
+declare function fastifyExampleCallback(...params: Parameters<FastifyExampleCallback>): ReturnType<FastifyExampleCallback>
+
+export default fastifyExampleCallback

types/plugin.d.ts

@@ -8,7 +8,9 @@ import {
   RawServerDefault,
   FastifyTypeProvider,
   FastifyTypeProviderDefault,
+  FastifyBaseLogger,
 } from 'fastify'
+import { IncomingMessage, Server, ServerResponse } from 'http'
 
 type FastifyPlugin = typeof fastifyPlugin
 
@@ -45,40 +47,16 @@ declare namespace fastifyPlugin {
  * @param fn Fastify plugin function
  * @param options Optional plugin options
  */
- declare function fastifyPlugin<
- Options extends FastifyPluginOptions,
- RawServer extends RawServerBase = RawServerDefault,
- TypeProvider extends FastifyTypeProvider = FastifyTypeProviderDefault
->(
- fn: FastifyPluginAsync<Options, RawServer, TypeProvider>,
- options?: fastifyPlugin.PluginMetadata
-): FastifyPluginAsync<Options, RawServer, TypeProvider>;
-
-declare function fastifyPlugin<
- Options extends FastifyPluginOptions,
- RawServer extends RawServerBase = RawServerDefault,
- TypeProvider extends FastifyTypeProvider = FastifyTypeProviderDefault
->(
- fn: FastifyPluginAsync<Options, RawServer, TypeProvider>,
- options?: string
-): FastifyPluginAsync<Options, RawServer, TypeProvider>;
-
-declare function fastifyPlugin<
- Options extends FastifyPluginOptions,
- RawServer extends RawServerBase = RawServerDefault,
- TypeProvider extends FastifyTypeProvider = FastifyTypeProviderDefault
->(
- fn: FastifyPluginCallback<Options, RawServer, TypeProvider>,
- options?: fastifyPlugin.PluginMetadata
-): FastifyPluginCallback<Options, RawServer, TypeProvider>;
 
 declare function fastifyPlugin<
- Options extends FastifyPluginOptions,
- RawServer extends RawServerBase = RawServerDefault,
- TypeProvider extends FastifyTypeProvider = FastifyTypeProviderDefault
+  Options extends FastifyPluginOptions = Record<never, never>,
+  RawServer extends RawServerBase = RawServerDefault,
+  TypeProvider extends FastifyTypeProvider = FastifyTypeProviderDefault,
+  Logger extends FastifyBaseLogger = FastifyBaseLogger,
+  Fn extends FastifyPluginCallback<Options, RawServer, TypeProvider, Logger> | FastifyPluginAsync<Options, RawServer, TypeProvider, Logger> = FastifyPluginCallback<Options, RawServer, TypeProvider, Logger>
 >(
- fn: FastifyPluginCallback<Options>,
- options?: string
-): FastifyPluginCallback<Options>;
+  fn: Fn extends unknown ? Fn extends (...args: any) => Promise<any> ? FastifyPluginAsync<Options, RawServer, TypeProvider, Logger> : FastifyPluginCallback<Options, RawServer, TypeProvider, Logger> : Fn,
+  options?: fastifyPlugin.PluginMetadata | string
+): Fn;
 
 export = fastifyPlugin

types/plugin.test-d.ts

@@ -1,8 +1,10 @@
 import fastifyPlugin from '..';
-import fastify, { FastifyPluginCallback, FastifyPluginAsync, FastifyError, FastifyInstance, FastifyPluginOptions } from 'fastify';
-import { expectAssignable } from 'tsd'
+import fastify, { FastifyPluginCallback, FastifyPluginAsync, FastifyError, FastifyInstance, FastifyPluginOptions, RawServerDefault, FastifyTypeProviderDefault, FastifyBaseLogger } from 'fastify';
+import { expectAssignable, expectError, expectNotType, expectType } from 'tsd'
 import { Server } from "node:https"
 import { TypeBoxTypeProvider } from "@fastify/type-provider-typebox"
+import fastifyExampleCallback from './example-callback.test-d';
+import fastifyExampleAsync from './example-async.test-d';
 
 interface Options {
   foo: string
@@ -13,79 +15,81 @@ const testSymbol = Symbol('foobar')
 // Callback
 
 const pluginCallback: FastifyPluginCallback = (fastify, options, next) => { }
-expectAssignable<FastifyPluginCallback>(fastifyPlugin(pluginCallback))
+expectType<FastifyPluginCallback>(fastifyPlugin(pluginCallback))
 
 const pluginCallbackWithTypes = (fastify: FastifyInstance, options: FastifyPluginOptions, next: (error?: FastifyError) => void): void => { }
 expectAssignable<FastifyPluginCallback>(fastifyPlugin(pluginCallbackWithTypes))
+expectNotType<any>(fastifyPlugin(pluginCallbackWithTypes))
 
 expectAssignable<FastifyPluginCallback>(fastifyPlugin((fastify: FastifyInstance, options: FastifyPluginOptions, next: (error?: FastifyError) => void): void => { }))
+expectNotType<any>(fastifyPlugin((fastify: FastifyInstance, options: FastifyPluginOptions, next: (error?: FastifyError) => void): void => { }))
 
-expectAssignable<FastifyPluginCallback>(fastifyPlugin(pluginCallback, '' ))
-expectAssignable<FastifyPluginCallback>(fastifyPlugin(pluginCallback, {
+expectType<FastifyPluginCallback>(fastifyPlugin(pluginCallback, ''))
+expectType<FastifyPluginCallback>(fastifyPlugin(pluginCallback, {
   fastify: '',
   name: '',
   decorators: {
-    fastify: [ '', testSymbol ],
-    reply: [ '', testSymbol ],
-    request: [ '', testSymbol ]
+    fastify: ['', testSymbol],
+    reply: ['', testSymbol],
+    request: ['', testSymbol]
   },
-  dependencies: [ '' ],
+  dependencies: [''],
   encapsulate: true
 }))
 
 const pluginCallbackWithOptions: FastifyPluginCallback<Options> = (fastify, options, next) => {
-  expectAssignable<string>(options.foo)
+  expectType<string>(options.foo)
 }
 
-expectAssignable<FastifyPluginCallback<Options>>(fastifyPlugin(pluginCallbackWithOptions))
+expectType<FastifyPluginCallback<Options>>(fastifyPlugin(pluginCallbackWithOptions))
 
 const pluginCallbackWithServer: FastifyPluginCallback<Options, Server> = (fastify, options, next) => {
-  expectAssignable<Server>(fastify.server)
+  expectType<Server>(fastify.server)
 }
 
-expectAssignable<FastifyPluginCallback<Options, Server>>(fastifyPlugin(pluginCallbackWithServer))
+expectType<FastifyPluginCallback<Options, Server>>(fastifyPlugin(pluginCallbackWithServer))
 
-const pluginCallbackWithTypeProvider: FastifyPluginCallback<Options, Server, TypeBoxTypeProvider> = (fastify, options, next) => {}
+const pluginCallbackWithTypeProvider: FastifyPluginCallback<Options, Server, TypeBoxTypeProvider> = (fastify, options, next) => { }
 
-expectAssignable<FastifyPluginCallback<Options, Server, TypeBoxTypeProvider>>(fastifyPlugin(pluginCallbackWithTypeProvider))
+expectType<FastifyPluginCallback<Options, Server, TypeBoxTypeProvider>>(fastifyPlugin(pluginCallbackWithTypeProvider))
 
 // Async
 
 const pluginAsync: FastifyPluginAsync = async (fastify, options) => { }
-expectAssignable<FastifyPluginAsync>(fastifyPlugin(pluginAsync))
+expectType<FastifyPluginAsync>(fastifyPlugin(pluginAsync))
 
 const pluginAsyncWithTypes = async (fastify: FastifyInstance, options: FastifyPluginOptions): Promise<void> => { }
-expectAssignable<FastifyPluginAsync>(fastifyPlugin(pluginAsyncWithTypes))
+expectType<FastifyPluginAsync<FastifyPluginOptions, RawServerDefault, FastifyTypeProviderDefault>>(fastifyPlugin(pluginAsyncWithTypes))
 
-expectAssignable<FastifyPluginAsync>(fastifyPlugin(async (fastify: FastifyInstance, options: FastifyPluginOptions): Promise<void> => { }))
-expectAssignable<FastifyPluginAsync>(fastifyPlugin(pluginAsync, '' ))
-expectAssignable<FastifyPluginAsync>(fastifyPlugin(pluginAsync, {
+expectType<FastifyPluginAsync<FastifyPluginOptions, RawServerDefault, FastifyTypeProviderDefault>>(fastifyPlugin(async (fastify: FastifyInstance, options: FastifyPluginOptions): Promise<void> => { }))
+expectType<FastifyPluginAsync>(fastifyPlugin(pluginAsync, ''))
+expectType<FastifyPluginAsync>(fastifyPlugin(pluginAsync, {
   fastify: '',
   name: '',
   decorators: {
-    fastify: [ '', testSymbol ],
-    reply: [ '', testSymbol ],
-    request: [ '', testSymbol ]
+    fastify: ['', testSymbol],
+    reply: ['', testSymbol],
+    request: ['', testSymbol]
   },
-  dependencies: [ '' ],
+  dependencies: [''],
   encapsulate: true
 }))
 
 const pluginAsyncWithOptions: FastifyPluginAsync<Options> = async (fastify, options) => {
-  expectAssignable<string>(options.foo)
+  expectType<string>(options.foo)
 }
 
-expectAssignable<FastifyPluginAsync<Options>>(fastifyPlugin(pluginAsyncWithOptions))
+expectType<FastifyPluginAsync<Options>>(fastifyPlugin(pluginAsyncWithOptions))
 
 const pluginAsyncWithServer: FastifyPluginAsync<Options, Server> = async (fastify, options) => {
-  expectAssignable<Server>(fastify.server)
+  expectType<Server>(fastify.server)
 }
 
-expectAssignable<FastifyPluginAsync<Options, Server>>(fastifyPlugin(pluginAsyncWithServer))
+expectType<FastifyPluginAsync<Options, Server>>(fastifyPlugin(pluginAsyncWithServer))
 
-const pluginAsyncWithTypeProvider: FastifyPluginAsync<Options, Server, TypeBoxTypeProvider> = async (fastify, options) => {}
+const pluginAsyncWithTypeProvider: FastifyPluginAsync<Options, Server, TypeBoxTypeProvider> = async (fastify, options) => { }
 
-expectAssignable<FastifyPluginAsync<Options, Server, TypeBoxTypeProvider>>(fastifyPlugin(pluginAsyncWithTypeProvider))
+expectType<FastifyPluginAsync<Options, Server, TypeBoxTypeProvider>>(fastifyPlugin(pluginAsyncWithTypeProvider))
 
 // Fastify register
 
@@ -100,3 +104,63 @@ server.register(fastifyPlugin(pluginAsyncWithTypes))
 server.register(fastifyPlugin(pluginAsyncWithOptions))
 server.register(fastifyPlugin(pluginAsyncWithServer))
 server.register(fastifyPlugin(pluginAsyncWithTypeProvider))
+
+// properly handling callback and async
+fastifyPlugin(function (fastify, options, next) {
+  expectType<FastifyInstance>(fastify)
+  expectType<Record<never, never>>(options)
+  expectType<(err?: Error) => void>(next)
+})
+
+fastifyPlugin<Options>(function (fastify, options, next) {
+  expectType<FastifyInstance>(fastify)
+  expectType<Options>(options)
+  expectType<(err?: Error) => void>(next)
+})
+
+fastifyPlugin<Options>(async function (fastify, options) {
+  expectType<FastifyInstance>(fastify)
+  expectType<Options>(options)
+})
+
+expectAssignable<FastifyPluginAsync<Options, RawServerDefault, FastifyTypeProviderDefault, FastifyBaseLogger>>(fastifyPlugin(async function (fastify: FastifyInstance, options: Options) { }))
+expectNotType<any>(fastifyPlugin(async function (fastify: FastifyInstance, options: Options) { }))
+
+fastifyPlugin(async function (fastify, options: Options) {
+  expectType<FastifyInstance>(fastify)
+  expectType<Options>(options)
+})
+
+fastifyPlugin(async function (fastify, options) {
+  expectType<FastifyInstance>(fastify)
+  expectType<Record<never, never>>(options)
+})
+
+expectError(
+  fastifyPlugin(async function (fastify, options: Options, next) {
+    expectType<FastifyInstance>(fastify)
+    expectType<Options>(options)
+  })
+)
+expectAssignable<FastifyPluginCallback<Options>>(fastifyPlugin(function (fastify, options, next) { }))
+expectNotType<any>(fastifyPlugin(function (fastify, options, next) { }))
+
+fastifyPlugin(function (fastify, options: Options, next) {
+  expectType<FastifyInstance>(fastify)
+  expectType<Options>(options)
+  expectType<(err?: Error) => void>(next)
+})
+
+expectError(
+  fastifyPlugin(function (fastify, options: Options, next) {
+    expectType<FastifyInstance>(fastify)
+    expectType<Options>(options)
+    return Promise.resolve()
+  })
+)
+
+server.register(fastifyExampleCallback, { foo: 'bar' })
+expectError(server.register(fastifyExampleCallback, { foo: 'baz' }))
+
+server.register(fastifyExampleAsync, { foo: 'bar' })
+expectError(server.register(fastifyExampleAsync, { foo: 'baz' }))