Add better error message for hoist_storage on extern stage (#8974)

Co-authored-by: Claude Code <noreply@anthropic.com>

Author: abadams

src/ScheduleFunctions.cpp

@@ -2317,6 +2317,12 @@ bool validate_schedule(Function f, const Stmt &s, const Target &target, bool is_
         user_error << "Func \"" << f.name() << "\" is scheduled with ring_buffer(), but has matching store_at and hoist_storage levels. Add an explicit hoist_storage directive to the schedule to fix the issue.\n";
     }
 
+    // Check if hoist_storage is used with an extern stage
+    if (f.has_extern_definition() && !hoist_storage_at.is_inlined() && hoist_storage_at != store_at) {
+        user_error << "Func \"" << f.name() << "\" is an extern function with storage hoisted to a different level than store_at. "
+                   << "Func::hoist_storage is not currently supported for extern functions.";
+    }
+
     vector<ComputeLegalSchedules::Site> &sites = legal.sites_allowed;
     int store_idx = -1, compute_idx = -1, hoist_storage_idx = -1;
     for (size_t i = 0; i < sites.size(); i++) {

test/error/CMakeLists.txt

@@ -66,6 +66,7 @@ tests(GROUPS error
       func_tuple_types_mismatch.cpp
       func_tuple_update_types_mismatch.cpp
       fuse_vectorized_var_with_rvar.cpp
+      hoist_storage_extern.cpp
       hoist_storage_without_compute_at.cpp
       implicit_args.cpp
       impossible_constraints.cpp

test/error/hoist_storage_extern.cpp

@@ -0,0 +1,29 @@
+#include "Halide.h"
+#include <stdio.h>
+
+using namespace Halide;
+
+int main(int argc, char **argv) {
+    Func f, g, h;
+    Var x, y;
+
+    // Extern stage
+    f.define_extern("make_data", {}, Int(32), 1);
+
+    // Two-stage pipeline
+    g(x, y) = f(2 * x + y);
+    h(x) = g(x, 0) + g(x, 1);
+
+    // Schedule with hoist_storage on an extern function
+    // This should produce an error because buffer metadata needed
+    // for device cleanup is only available at the store_at location
+    h.compute_root().vectorize(x, 8, TailStrategy::RoundUp);
+    g.compute_at(h, x).unroll(y).vectorize(x);
+    f.hoist_storage(g, Var::outermost()).bound_storage(_0, 16).compute_at(g, y);
+
+    // This should fail with a user error
+    h.compile_jit();
+
+    printf("Success!\n");
+    return 0;
+}