Improve QASM3 parameter export API

Author: GoiBasia

releasenotes/notes/qasm3-exporter-parameter-146.yaml

@@ -7,7 +7,12 @@ features:
     `QuantumCircuit::to_qasm3()` and `Qiskit::qasm3::dumps(circuit)` use
     `QuantumCircuit::parameter_symbols()` to emit original parameter symbol
     names by default.  `Qiskit::qasm3::dumps(circuit, parameter_names)` remains
-    available for callers that need to provide names explicitly.
+    available for callers that need to provide names explicitly. Explicit
+    parameter names must be valid OpenQASM identifiers and must not conflict
+    with reserved OpenQASM or standard-gate identifiers.
 
-    Until Qiskit C exposes parameter UUIDs to C++, circuits cannot contain
-    distinct `Parameter` objects with the same symbol name.
+    `QuantumCircuit::parameter_symbols()` currently derives names from
+    instruction parameter expression strings and cross-checks the recovered
+    count against Qiskit C.  Until Qiskit C exposes canonical parameter names
+    or UUIDs to C++, circuits cannot contain distinct `Parameter` objects with
+    the same symbol name.

src/circuit/quantumcircuit_def.hpp

@@ -109,7 +109,7 @@ class QuantumCircuit
 	{
 		static const char *function_names[] = {
 			"abs", "acos", "asin", "atan", "conj", "conjugate", "cos",
-			"exp", "log", "sign", "sin", "tan"
+			"exp", "log", "sign", "sin", "sqrt", "tan"
 		};
 		return std::find(function_names, function_names + sizeof(function_names) / sizeof(function_names[0]), name)
 			!= function_names + sizeof(function_names) / sizeof(function_names[0]);