Fix null coal assign

src/typing/typer.ml

@@ -1877,10 +1877,16 @@ and type_expr ?(mode=MGet) ctx (e,p) (with_type:WithType.t) =
 		let e_cond = mk (TBinop(OpNotEq,e1,e_null)) ctx.t.tbool e1.epos in
 		let e_if = mk (TIf(e_cond,cast e1,Some e2)) iftype p in
 		vr#to_texpr e_if
-	| EBinop (OpAssignOp OpNullCoal,e1,e2) ->
-		let e_cond = EBinop(OpNotEq,e1,(EConst(Ident "null"), p)) in
-		let e_if = EIf ((e_cond, p),e1,Some e2) in
-		type_assign ctx e1 (e_if, p) with_type p
+	| EBinop (OpAssignOp OpNullCoal,e1_orig,e2) ->
+		let vr = new value_reference ctx in
+		let e1 = type_expr ctx (Expr.ensure_block e1_orig) with_type in
+		let e1_null_t = if is_nullable e1.etype then e1.etype else ctx.t.tnull e1.etype in
+		let e1_var = vr#as_var (Option.default "tmp" (WithType.get_expected_name with_type)) {e1 with etype = e1_null_t} in
+		let e_null = Builder.make_null e1_null_t e1.epos in
+		let e_cond = mk (TBinop(OpNotEq,e1_var,e_null)) ctx.t.tbool e1.epos in
+		let e_assign = type_assign ctx e1_orig (Expr.ensure_block e2) with_type p in
+		let e_if = mk (TIf(e_cond,e1_var,Some e_assign)) e1.etype p in
+		vr#to_texpr e_if
 	| EBinop (op,e1,e2) ->
 		type_binop ctx op e1 e2 false with_type p
 	| EBlock [] when (match with_type with

tests/optimization/src/issues/Issue11931.hx

@@ -0,0 +1,25 @@
+package issues;
+
+class Issue11931 {
+	@:js('
+		var arr = [];
+		var _g = 0;
+		var _g1 = issues_Issue11931.test_i;
+		while(_g < _g1) {
+			var x = _g++;
+			var e = arr[x];
+			issues_Issue11931.use(e != null ? e : arr[x] = []);
+		}
+	')
+	static function test() {
+		static var i = 0;
+		var arr:Array<Array<Int>> = [];
+		for (x in 0...i) {
+			var e = arr[x] ??= [];
+			use(e);
+		}
+	}
+
+	@:pure(false)
+	static function use(v:Array<Int>) {}
+}