Fix Target.instruction_supported when target.num_qubits == None (backport #13655)

crates/accelerate/src/target_transpiler/mod.rs

@@ -1151,6 +1151,12 @@ impl Target {
 
     /// Checks whether an instruction is supported by the Target based on instruction name and qargs.
     pub fn instruction_supported(&self, operation_name: &str, qargs: Option<&Qargs>) -> bool {
+        // Handle case where num_qubits is None by checking globally supported operations
+        let qargs: Option<&Qargs> = if self.num_qubits.is_none() {
+            None
+        } else {
+            qargs
+        };
         if self.gate_map.contains_key(operation_name) {
             if let Some(_qargs) = qargs {
                 let qarg_set: HashSet<&PhysicalQubit> = _qargs.iter().collect();

releasenotes/notes/fix-target-instr-supported-900a1caa76e30655.yaml

@@ -0,0 +1,6 @@
+---
+fixes:
+  - |
+    Fixed a bug in the :meth:`.Target.instruction_supported` method where 
+    targets with ``self.num_qubits==None`` would always return ``false`` 
+    independently of the  supported basis set. 

test/python/transpiler/test_target.py

@@ -1169,6 +1169,14 @@ def test_instruction_supported_no_args(self):
     def test_instruction_supported_no_operation(self):
         self.assertFalse(self.ibm_target.instruction_supported(qargs=(0,), parameters=[math.pi]))
 
+    def test_instruction_supported_no_qubits(self):
+        """Checks that instruction supported works when target.num_qubits is None."""
+        target = Target.from_configuration(["u", "cx", "rxx"])
+        self.assertTrue(target.instruction_supported("u", (0,)))
+        self.assertTrue(target.instruction_supported("cx", (0, 1)))
+        self.assertTrue(target.instruction_supported("cx", None))
+        self.assertTrue(target.instruction_supported("rxx", (2, 3)))
+
     def test_target_serialization_preserve_variadic(self):
         """Checks that variadics are still seen as variadic after serialization"""