optimize away double bolean negation (closes #9239)

src/optimization/optimizer.ml

@@ -133,6 +133,9 @@ let sanitize_expr com e =
 		let e1 = if loop e1 true then parent e1 else e1 in
 		let e2 = if loop e2 false then parent e2 else e2 in
 		{ e with eexpr = TBinop (op,e1,e2) }
+	| TUnop (Not,Prefix,{ eexpr = (TUnop (Not,Prefix,e1)) | (TParenthesis { eexpr = TUnop (Not,Prefix,e1) }) })
+		when ExtType.is_bool (Abstract.follow_with_abstracts e1.etype) ->
+		e1
 	| TUnop (op,mode,e1) ->
 		let rec loop ee =
 			match ee.eexpr with

tests/optimization/src/TestJs.hx

@@ -585,6 +585,23 @@ class TestJs {
 		}
 		f(10);
 	}
+
+	@:js('
+		var x = function() {return true;};
+		var f = function(b) {
+			if(x()) {b = true;}
+			TestJs.use(b);
+		};
+		f(false);
+	')
+	static function testIssue9239_noDoubleNegation() {
+		function x() return true;
+		function f(b:Bool) {
+			b = x() || b;
+			TestJs.use(b);
+		}
+		f(false);
+	}
 }
 
 extern class Extern {