Fixed silent ignore of legacy predict flags (#12211)

R-package/DESCRIPTION

@@ -71,6 +71,6 @@ Imports:
     data.table (>= 1.9.6),
     jsonlite (>= 1.0)
 Roxygen: list(markdown = TRUE)
-RoxygenNote: 7.3.3
 Encoding: UTF-8
 SystemRequirements: GNU make, C++17
+Config/roxygen2/version: 8.0.0

R-package/R/xgboost.R

@@ -1321,7 +1321,9 @@ xgboost <- function(
 #'
 #' Note that this check might add some sizable latency to the predictions, so it's
 #' recommended to disable it for performance-sensitive applications.
-#' @param ... Not used.
+#' @param ... Legacy boolean flags (e.g., \code{predcontrib}) are supported for
+#'   backward compatibility but are deprecated and map to the \code{type} argument.
+#'   Any other arguments passed to \code{...} will cause an error.
 #' @return Either a numeric vector (for 1D outputs), numeric matrix (for 2D outputs), numeric array
 #' (for 3D and higher), or `factor` (for class predictions). See documentation for parameter `type`
 #' for details about what the output type and shape will be.
@@ -1352,6 +1354,47 @@ predict.xgboost <- function(
   validate_features = TRUE,
   ...
 ) {
+  dots <- list(...)
+  if (length(dots) > 0) {
+    mapping <- list(
+      outputmargin = "raw",
+      predleaf = "leaf",
+      predcontrib = "contrib",
+      approxcontrib = "contrib",
+      predinteraction = "interaction"
+    )
+    found_legacy <- intersect(names(dots), names(mapping))
+
+    if (length(found_legacy) > 0) {
+      for (legacy_arg in found_legacy) {
+        if (isTRUE(dots[[legacy_arg]])) {
+          type <- mapping[[legacy_arg]]
+          warning(
+            sprintf(
+              "Argument '%s' is deprecated. Please use type = '%s' instead.",
+              legacy_arg, type
+            ),
+            call. = FALSE
+          )
+          break
+        }
+      }
+      dots[found_legacy] <- NULL
+    }
+
+    if (length(dots) > 0) {
+      dot_names <- names(dots)
+      if (is.null(dot_names)) {
+        dot_names <- rep("<unnamed>", length(dots))
+      }
+      dot_names[dot_names == ""] <- "<unnamed>"
+      stop(
+        "predict.xgboost: arguments in '...' are not supported (",
+        paste(dot_names, collapse = ", "), ")."
+      )
+    }
+  }
+
   if (inherits(newdata, "xgb.DMatrix")) {
     stop(
       "Predictions on 'xgb.DMatrix' objects are not supported with 'xgboost' class.",

R-package/tests/testthat/test_xgboost.R

@@ -1131,3 +1131,53 @@ test_that("Linear booster importance uses class names", {
   expect_true(is.factor(imp$Class))
   expect_equal(levels(imp$Class), levels(y))
 })
+
+test_that("predict.xgboost maps legacy boolean flags to type", {
+  y <- mtcars$mpg
+  x <- as.matrix(mtcars[, -1L])
+  model <- xgboost(x, y, nthreads = 1L, nrounds = 1L)
+
+  expect_warning(
+    pred <- predict(model, x, predcontrib = TRUE),
+    paste0(
+      "Argument 'predcontrib' is deprecated. ",
+      "Please use type = 'contrib' instead."
+    ),
+    fixed = TRUE
+  )
+  expect_equal(dim(pred), c(nrow(x), ncol(x) + 1L))
+
+  # Test multiple flags (first is FALSE, second is TRUE)
+  expect_warning(
+    pred2 <- predict(model, x, outputmargin = FALSE, predinteraction = TRUE),
+    paste0(
+      "Argument 'predinteraction' is deprecated. ",
+      "Please use type = 'interaction' instead."
+    ),
+    fixed = TRUE
+  )
+  expect_equal(dim(pred2), c(nrow(x), ncol(x) + 1L, ncol(x) + 1L))
+
+  # Test conflict between legacy flag and explicit type
+  expect_warning(
+    pred3 <- predict(model, x, type = "response", approxcontrib = TRUE),
+    paste0(
+      "Argument 'approxcontrib' is deprecated. ",
+      "Please use type = 'contrib' instead."
+    ),
+    fixed = TRUE
+  )
+  expect_equal(dim(pred3), c(nrow(x), ncol(x) + 1L))
+
+  # Test unsupported arguments
+  expect_error(
+    predict(model, x, foobar = TRUE),
+    "predict.xgboost: arguments in '...' are not supported (foobar).",
+    fixed = TRUE
+  )
+  expect_error(
+    predict(model, x, "response", NULL, NULL, TRUE, "some_unnamed_arg"),
+    "predict.xgboost: arguments in '...' are not supported (<unnamed>).",
+    fixed = TRUE
+  )
+})