fix: add receive‑operand checks for declarations and extended type handling

compiler/crates/hy-check/src/emit_guard.rs

@@ -1310,7 +1310,9 @@ impl<'a> Checker<'a> {
         // TypeRef, so running the builtin-argument check here covers struct
         // fields, aliases, local annotations, literals and conversions.
         let empty_st = crate::model::FnState::default();
-        self.check_expr_builtin_args(st.unwrap_or(&empty_st), len);
+        let st_ref = st.unwrap_or(&empty_st);
+        self.check_expr_recv_operands(st_ref, len);
+        self.check_expr_builtin_args(st_ref, len);
         let is_wrapped_ident_int_conv = matches!(
             unwrap_parens(len),
             Expr::Call { callee, args, .. }

compiler/crates/hy-check/src/recv.rs

@@ -199,7 +199,7 @@ impl<'a> Checker<'a> {
     /// Recurse an expression tree, classifying every `Expr::Receive`
     /// operand found at any depth. FnLit bodies are the A5 perimeter —
     /// not walked here (the closure pass enters them).
-    fn check_expr_recv_operands(&mut self, st: &FnState, expr: &Expr) {
+    pub(crate) fn check_expr_recv_operands(&mut self, st: &FnState, expr: &Expr) {
         match expr {
             Expr::Receive { chan, span } => {
                 self.check_recv_operand(st, chan, *span);

compiler/crates/hy-check/tests/recv_operand_soundness.rs

@@ -196,3 +196,46 @@ fn drain(opt option[chan chan<- int]) {
     );
     assert_has(&r_select, "HYB-E3008");
 }
+
+#[test]
+fn decl_type_expr_array_length_non_channel_receive_rejected_3880() {
+    // Issue #3880: struct field, type alias, enum payload, and fn signature
+    // array-length TypeExprs must reject receive from non-channel with HYB-E2008.
+    let r_struct = check(
+        r#"package main
+
+struct bad_struct {
+    a [<-5]int
+}
+"#,
+    );
+    assert_has(&r_struct, "HYB-E2008");
+
+    let r_alias = check(
+        r#"package main
+
+type bad_alias = [<-"abc"]int
+"#,
+    );
+    assert_has(&r_alias, "HYB-E2008");
+
+    let r_fn_param = check(
+        r#"package main
+
+fn run(a [<-5]int) {
+    _ = a
+}
+"#,
+    );
+    assert_has(&r_fn_param, "HYB-E2008");
+
+    let r_fn_ret = check(
+        r#"package main
+
+fn run() [<-5]int {
+    return [0]
+}
+"#,
+    );
+    assert_has(&r_fn_ret, "HYB-E2008");
+}