input and output declarations are unusable #394

Limitation

Input/output defines input and output
modifiers on classical declarations. input marks a value supplied at run time — the spec's
motivating case is a parameterised circuit compiled once and executed many times with
different angles. output marks a value returned to the host.

pyqasm parses both declarations but never registers them in scope. Any later use fails.

Example QASM failure

The spec's own parameterised-circuit pattern:

OPENQASM 3.0;
include "stdgates.inc";
input float[64] a;
qubit[1] q;
rx(a) q[0];
// ValidationError: Invalid parameter 'a' for gate 'rx'

An input used in ordinary classical code fails the same way:

OPENQASM 3.0;
input int[8] a;
int[8] b = a;
// ValidationError: Invalid initialization value for variable 'b'

output is not registered either:

OPENQASM 3.0;
output bit[2] c;
qubit[2] q;
c = measure q;
// ValidationError: Missing register declaration for 'c' in measurement operation

OPENQASM 3.0;
output int[8] c;
c = 1;
// ValidationError: Undefined variable c in assignment

A bare input bit[2] a; with no subsequent use validates cleanly, which confirms the
declaration is parsed and then discarded.

Change Requested

1. Register input and output declarations in the global scope with their declared types,
   so that every later reference resolves.
2. An output-qualified variable must be assignable, including as a measurement target.
3. Preserve both qualifiers through dumps() — a round-trip must not silently turn an input
   into an ordinary declaration.
4. Enforce the spec's scope rule: input and output are permitted only in global scope.

Open design question

pyqasm's unroller evaluates classical expressions to concrete values, but an input has no
value at analysis time. Two viable directions, and this issue should settle which:

• Binding API — accept values at load or unroll time, e.g.
  pyqasm.loads(qasm, inputs={"a": 0.5}), and substitute them during unrolling. Concrete
  output, no representation change, but requires the caller to supply every input.
• Symbolic passthrough — treat an unbound input as an opaque symbol, validate its type
  and usage, and emit the expression unevaluated. Keeps the program parameterised at the cost
  of expression handling that tolerates non-numeric operands.

Supporting both — bind when a value is given, pass through when it is not — is likely the
right end state, but the first increment should pick one. Maintainer input needed.

Implementation Details

• openqasm3 exposes the qualifier as IOKeyword.input / IOKeyword.output on
  ClassicalDeclaration / IODeclaration. Confirm which node type the parser produces for
  each so the visitor branches correctly.
• The declaration path is Qasm3Visitor._visit_classical_declaration in
  src/pyqasm/visitor.py; the IODeclaration node appears to fall through without a scope
  entry. Registering the variable is the minimal fix that clears all four failures above.
• The error "Invalid parameter 'a' for gate 'rx'" comes from the gate-argument evaluation
  path — it fires because the identifier does not resolve, so scope registration fixes it
  directly under the binding approach, and requires expression-level tolerance under the
  symbolic approach.
• output needs the variable marked assignable and visible to the measurement path that
  raises "Missing register declaration for 'c' in measurement operation".
• Tests: tests/qasm3/test_declarations.py — input in a gate argument, input in classical
  arithmetic, output as a measurement target, output in a plain assignment, a dumps()
  round-trip preserving both keywords, and input declared inside a def body raising a
  ValidationError.