Test the ifelse() rank-mismatch error, drop an unreachable guard

Coverage flagged both stop() calls in check_ifelse_branch_shape() as
untested.

The rank check is reachable: a matrix branch under a vector `test`, or
the reverse, hits it, and the existing mismatch test only exercises the
per-axis verdict. Add a case for each direction.

The `is.null(hoist)` arm is not reachable. r2f() replaces a NULL hoist
with a fresh one before dispatching to any handler, and the only other
dispatch route resolves `f<-`-style names, so the ifelse() handler
always has a hoist to emit the size guard into. Delete it and record
the invariant in the function's comment rather than testing dead code.

Author: mns-nordicals

R/r2f-conditionals.R

@@ -33,7 +33,9 @@ ifelse_axis_verdict <- function(test_dim, branch_dim) {
 # natively; a non-scalar branch must match `test`'s shape, because
 # merge() requires conformable arguments and a runtime mismatch would
 # read past the shorter branch. Statically unequal dims are a compile
-# error; symbolic dims get a statement-level runtime size guard.
+# error; symbolic dims get a statement-level runtime size guard, emitted into
+# `hoist` -- always a live hoist context, since r2f() substitutes a fresh one
+# before dispatching to any handler.
 check_ifelse_branch_shape <- function(branch, mask, hoist, scope) {
   if (passes_as_scalar(branch@value)) {
     return(invisible())
@@ -57,13 +59,6 @@ check_ifelse_branch_shape <- function(branch, mask, hoist, scope) {
   if (!length(unknown_axes)) {
     return(invisible())
   }
-  if (is.null(hoist)) {
-    stop(
-      "cannot emit a runtime length guard here; ",
-      "ifelse() branch lengths must match `test` statically",
-      call. = FALSE
-    )
-  }
   # size() is an inquiry, so applying it to operand expression text does
   # not evaluate the operands.
   condition <- str_flatten(

tests/testthat/test-ifelse.R

@@ -59,6 +59,23 @@ test_that("ifelse with statically mismatched branch lengths is a compile error",
   expect_error(quick(fn), "R-style recycling is not supported")
 })
 
+test_that("ifelse with a branch of different rank than test is a compile error", {
+  # merge() requires conformable arguments: a matrix branch under a vector
+  # `test` is R recycling, not broadcasting.
+  fn <- function(c, m) {
+    declare(type(c = logical(3)), type(m = double(3, 3)))
+    ifelse(c, m, 0)
+  }
+  expect_error(quick(fn), "R-style recycling is not supported")
+
+  # the mirror image, and in `no` position: a vector branch under a matrix test
+  fn2 <- function(c, a) {
+    declare(type(c = logical(2, 2)), type(a = double(4)))
+    ifelse(c, 0, a)
+  }
+  expect_error(quick(fn2), "R-style recycling is not supported")
+})
+
 test_that("ifelse guards unknown branch lengths at runtime", {
   fn <- function(c, a, b) {
     declare(type(c = logical(NA)), type(a = double(NA)), type(b = double(NA)))