fix

vlib/v/gen/c/fn.v

@@ -1563,6 +1563,11 @@ fn (mut g Gen) method_call(node ast.CallExpr) {
 			g.call_args(node)
 		}
 		g.write(')')
+		if !node.return_type.has_option_or_result() {
+			if g.table.final_sym(node.return_type).kind == .array_fixed {
+				g.write('.ret_arr')
+			}
+		}
 		return
 	}
 	left_sym := g.table.sym(left_type)

vlib/v/tests/fns/iface_method_fixed_arr_test.v

@@ -0,0 +1,41 @@
+module main
+
+pub type Mat4 = [16]f32
+
+pub fn (a Mat4) * (b Mat4) Mat4 {
+	mut res := Mat4{}
+	for i := 0; i < 4; i++ {
+		for j := 0; j < 4; j++ {
+			res[j * 4 + i] = 0
+			for k := 0; k < 4; k++ {
+				res[j * 4 + i] += a[k * 4 + i] * b[j * 4 + k]
+			}
+		}
+	}
+	return res
+}
+
+interface IGameObject {
+	parent    ?&IGameObject
+	transform Mat4
+	world_transform() Mat4
+}
+
+struct GameObject implements IGameObject {
+mut:
+	transform Mat4
+	parent    ?&IGameObject
+}
+
+fn (gameobject GameObject) world_transform() Mat4 {
+	if mut p := gameobject.parent {
+		return p.world_transform() * gameobject.transform
+	}
+	return gameobject.transform
+}
+
+fn test_main() {
+	t := GameObject{}
+	a := dump(t.world_transform())
+	assert a.len == 16
+}