fix: clearer error when wrt arg is a plain R literal

R/reverse.R

@@ -173,6 +173,18 @@ compute_requirements <- function(graph, wrt) {
   # `graph$inputs`; filter them out. `gradient()` already rejects static
   # `wrt` entries, so this drop is safe.
   is_static <- graph$is_static_flat
+  if (!is.null(is_static) && any(requires_grad_all & is_static)) {
+    flat_argnames <- rep(
+      graph$in_tree$names,
+      times = vapply(graph$in_tree$nodes, tree_size, integer(1L))
+    )
+    bad <- unique(flat_argnames[requires_grad_all & is_static])
+    cli_abort(c(
+      "Cannot compute gradient with respect to {.arg {bad}}.",
+      x = "{cli::qty(length(bad))}{?It was/They were} passed as {?a plain R value/plain R values}",
+      i = "{cli::qty(length(bad))}Pass {?it/them} as an {.cls AnvlArray}."
+    ))
+  }
   requires_grad <- if (is.null(is_static)) {
     requires_grad_all
   } else {

tests/testthat/_snaps/reverse.md

@@ -47,3 +47,23 @@
       ! Can only compute gradient with respect to float arrays.
       x Got i32
 
+# wrt arg passed as plain R literal errors clearly
+
+    Code
+      jit(function() gradient(nv_log, wrt = "operand")(1))()
+    Condition
+      Error in `compute_requirements()`:
+      ! Cannot compute gradient with respect to `operand`.
+      x It was passed as a plain R value
+      i Pass it as an <AnvlArray>.
+
+---
+
+    Code
+      jit(function() gradient(function(x, y) prim_add(x, y))(1, 2))()
+    Condition
+      Error in `compute_requirements()`:
+      ! Cannot compute gradient with respect to `x` and `y`.
+      x They were passed as plain R values
+      i Pass them as an <AnvlArray>.
+

tests/testthat/test-reverse.R

@@ -266,6 +266,15 @@ test_that("can only compute gradient w.r.t. float arrays", {
   })
 })
 
+test_that("wrt arg passed as plain R literal errors clearly", {
+  expect_snapshot(error = TRUE, {
+    jit(function() gradient(nv_log, wrt = "operand")(1))()
+  })
+  expect_snapshot(error = TRUE, {
+    jit(function() gradient(function(x, y) prim_add(x, y))(1, 2))()
+  })
+})
+
 test_that("can differentiate through integer/bool functions", {
   f <- function(x) {
     x1 <- nv_convert(x, "i32")