checker: fix returning error in if expr (#17783)

Author: yuyi98

vlib/v/checker/if.v

@@ -327,7 +327,7 @@ fn (mut c Checker) if_expr(mut node ast.IfExpr) ast.Type {
 								node.pos)
 						}
 					}
-				} else if !node.is_comptime {
+				} else if !node.is_comptime && stmt !is ast.Return {
 					c.error('`${if_kind}` expression requires an expression as the last statement of every branch',
 						branch.pos)
 				}

vlib/v/tests/return_error_in_if_expr_test.v

@@ -0,0 +1,25 @@
+struct NotFoundError {
+	Error
+}
+
+fn get_username() !string {
+	return NotFoundError{}
+}
+
+fn print_username() !string {
+	username := get_username() or {
+		if err is NotFoundError {
+			'test'
+		} else {
+			return err
+		}
+	}
+
+	println(username)
+	return username
+}
+
+fn test_return_err_in_if_expr() {
+	ret := print_username()!
+	assert ret == 'test'
+}