[js] fix type inference for Promise callback arguments (closes #6790)

std/js/Promise.hx

@@ -25,21 +25,21 @@ package js;
 import haxe.extern.EitherType;
 
 /**
-	The Promise object represents the eventual completion (or failure) of an 
+	The Promise object represents the eventual completion (or failure) of an
 	asynchronous operation and its resulting value.
-	
+
 	Documentation [Promise](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Promise) by [Mozilla Contributors](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Promise$history), licensed under [CC-BY-SA 2.5](https://creativecommons.org/licenses/by-sa/2.5/).
 **/
 @:native("Promise")
 extern class Promise<T>
 {
 	/**
-		Returns a Promise object that is resolved with the given value. If the 
-		value is Thenable, the returned promise will "follow" that 
-		thenable, adopting its eventual state; 
-		otherwise the returned promise will be fulfilled with the value. 
-		Generally, when it's unknown when value is a promise or not, 
-		use `Promise.resolve(value)` instead and work with the return value as 
+		Returns a Promise object that is resolved with the given value. If the
+		value is Thenable, the returned promise will "follow" that
+		thenable, adopting its eventual state;
+		otherwise the returned promise will be fulfilled with the value.
+		Generally, when it's unknown when value is a promise or not,
+		use `Promise.resolve(value)` instead and work with the return value as
 		a promise.
 	**/
 	@:overload(function<T>(promise : Promise<T>) : Promise<T> {})
@@ -52,20 +52,20 @@ extern class Promise<T>
 	static function reject<T>( ?reason : Dynamic ) : Promise<T>;
 
 	/**
-		Returns a promise that either fulfills when all of the promises in the 
+		Returns a promise that either fulfills when all of the promises in the
 		iterable argument have fulfilled or rejects as soon as one of the
-		promises in the iterable argument rejects. If the returned promise 
-		fulfills, it is fulfilled with an array of the values from the 
-		fulfilled promises in the same order as defined in the iterable. 
-		If the returned promise rejects, it is rejected with the reason from 
-		the first promise in the iterable that rejected. This method can be 
+		promises in the iterable argument rejects. If the returned promise
+		fulfills, it is fulfilled with an array of the values from the
+		fulfilled promises in the same order as defined in the iterable.
+		If the returned promise rejects, it is rejected with the reason from
+		the first promise in the iterable that rejected. This method can be
 		useful for aggregating results of multiple promises.
 	**/
 	static function all( iterable : Array<Dynamic> ) : Promise<Array<Dynamic>>;
 
 	/**
-		Returns a promise that fulfills or rejects as soon as one of the 
-		promises in the iterable fulfills or rejects, with the value or reason 
+		Returns a promise that fulfills or rejects as soon as one of the
+		promises in the iterable fulfills or rejects, with the value or reason
 		from that promise.
 	**/
 	static function race( iterable : Array<Dynamic> ) : Promise<Dynamic>;
@@ -74,26 +74,31 @@ extern class Promise<T>
 	function new( init : (resolve : (value : T) -> Void, reject: (reason : Dynamic) -> Void) -> Void ) : Void;
 
 	/**
-		Appends fulfillment and rejection handlers to the promise and returns a 
-		new promise resolving to the return value of the called handler, or to 
-		its original settled value if the promise was not handled 
+		Appends fulfillment and rejection handlers to the promise and returns a
+		new promise resolving to the return value of the called handler, or to
+		its original settled value if the promise was not handled
 		(i.e. if the relevant handler onFulfilled or onRejected is not a function).
 	**/
-	function then<TOut>( fulfillCallback : Null<PromiseCallback<T, TOut>>, ?rejectCallback : EitherType<Dynamic -> Void, PromiseCallback<Dynamic, TOut>> ) : Promise<TOut>;
+	function then<TOut>( fulfillCallback : Null<PromiseHandler<T, TOut>>, ?rejectCallback : PromiseHandler<Dynamic, TOut> ) : Promise<TOut>;
 
 	/**
-		Appends a rejection handler callback to the promise, and returns a new 
-		promise resolving to the return value of the callback if it is called, 
+		Appends a rejection handler callback to the promise, and returns a new
+		promise resolving to the return value of the callback if it is called,
 		or to its original fulfillment value if the promise is instead fulfilled.
 	**/
 	@:native("catch")
-	function catchError<TOut>( rejectCallback : EitherType<Dynamic -> Void, PromiseCallback<Dynamic, TOut>> ) : Promise<TOut>;
+	function catchError<TOut>( rejectCallback : PromiseHandler<Dynamic, TOut> ) : Promise<TOut>;
 }
 
 /**
-	Callback for the Promise object.
+	Handler type for the Promise object.
 **/
-typedef PromiseCallback<T, TOut> = EitherType<T -> TOut, T -> Promise<TOut>>;
+abstract PromiseHandler<T,TOut>(T->Dynamic) // T->Dynamic, so the compiler always knows the type of the argument and can infer it for then/catch callbacks
+	from T->Promise<TOut> // direct casts have priority, so we use that to prioritize the Promise<TOut> return type
+{
+	// function casts are checked after direct ones, so we place T->TOut here to make it have lower priority than T->Promise<TOut>
+	@:from static inline function fromNonPromise<T,TOut>(f:T->TOut):PromiseHandler<T,TOut> return cast f;
+}
 
 /**
 	A value with a `then` method.

tests/misc/projects/Issue6790/Main.hx

@@ -0,0 +1,24 @@
+import js.Promise;
+
+class Main {
+	static function main() {
+		var p = new Promise<String>(null);
+
+		$type(p.then(function(x) $type(x)));
+		$type(p.then(function(x) $type(x), function(e) $type(e)));
+
+		$type(p.then(function(x) {$type(x); return 1;}));
+		$type(p.then(function(x) {$type(x); return Promise.resolve(1);}));
+
+		$type(p.then(null,function(x) {$type(x); return 1;}));
+		$type(p.then(null,function(x) {$type(x); return Promise.resolve(1);}));
+
+		$type(p.then(function(x) {$type(x); return 1;}, function(e) {$type(e); return 1;}));
+		$type(p.then(function(x) {$type(x); return Promise.resolve(1);}, function(e) {$type(e); return 1;}));
+		$type(p.then(function(x) {$type(x); return 1;}, function(e) {$type(e); return Promise.resolve(1);}));
+		$type(p.then(function(x) {$type(x); return Promise.resolve(1);}, function(e) {$type(e); return Promise.resolve(1);}));
+
+		$type(p.catchError(function(x) {$type(x);}));
+		$type(p.catchError(function(x) {$type(x); return Promise.resolve(1);}));
+	}
+}

tests/misc/projects/Issue6790/Mismatch.hx

@@ -0,0 +1,8 @@
+import js.Promise;
+
+class Mismatch {
+	static function main() {
+		var p = new Promise<String>(null);
+		p.then(x -> 10, e -> "");
+	}
+}

tests/misc/projects/Issue6790/compile-fail.hxml

@@ -0,0 +1,3 @@
+-main Mismatch
+-js whatever.js
+--no-output

tests/misc/projects/Issue6790/compile-fail.hxml.stderr

@@ -0,0 +1,2 @@
+Mismatch.hx:6: characters 19-26 : (e : Unknown<0>) -> String should be Null<js.PromiseHandler<Dynamic, Int>>
+Mismatch.hx:6: characters 19-26 : For optional function argument 'rejectCallback'

tests/misc/projects/Issue6790/compile.hxml

@@ -0,0 +1,3 @@
+-main Main
+-js whatever.js
+--no-output

tests/misc/projects/Issue6790/compile.hxml.stderr

@@ -0,0 +1,29 @@
+Main.hx:7: characters 34-35 : Warning : String
+Main.hx:7: characters 9-37 : Warning : js.Promise<Void>
+Main.hx:8: characters 34-35 : Warning : String
+Main.hx:8: characters 56-57 : Warning : Unknown<0>
+Main.hx:8: characters 9-59 : Warning : js.Promise<Void>
+Main.hx:10: characters 35-36 : Warning : String
+Main.hx:10: characters 9-50 : Warning : js.Promise<Int>
+Main.hx:11: characters 35-36 : Warning : String
+Main.hx:11: characters 9-67 : Warning : js.Promise<Int>
+Main.hx:13: characters 40-41 : Warning : Unknown<0>
+Main.hx:13: characters 9-55 : Warning : js.Promise<Int>
+Main.hx:14: characters 40-41 : Warning : Unknown<0>
+Main.hx:14: characters 9-72 : Warning : js.Promise<Int>
+Main.hx:16: characters 35-36 : Warning : String
+Main.hx:16: characters 70-71 : Warning : Unknown<0>
+Main.hx:16: characters 9-85 : Warning : js.Promise<Int>
+Main.hx:17: characters 35-36 : Warning : String
+Main.hx:17: characters 87-88 : Warning : Unknown<0>
+Main.hx:17: characters 9-102 : Warning : js.Promise<Int>
+Main.hx:18: characters 35-36 : Warning : String
+Main.hx:18: characters 70-71 : Warning : Unknown<0>
+Main.hx:18: characters 9-102 : Warning : js.Promise<Int>
+Main.hx:19: characters 35-36 : Warning : String
+Main.hx:19: characters 87-88 : Warning : Unknown<0>
+Main.hx:19: characters 9-119 : Warning : js.Promise<Int>
+Main.hx:21: characters 41-42 : Warning : Unknown<0>
+Main.hx:21: characters 9-46 : Warning : js.Promise<Void>
+Main.hx:22: characters 41-42 : Warning : Unknown<0>
+Main.hx:22: characters 9-73 : Warning : js.Promise<Int>