Raise error on invalid qasm2 if statement then body (#248)

Author: david-pl

pyproject.toml

@@ -17,8 +17,8 @@ dependencies = [
     "rich>=13.9.4",
     "pydantic>=1.3.0,<2.11.0",
     "pandas>=2.2.3",
-    "pyqrack>=1.38.2 ; sys_platform == 'darwin'",
-    "pyqrack-cpu>=1.38.2 ; sys_platform != 'darwin'",
+    "pyqrack>=1.38.2,<1.41; sys_platform == 'darwin'",
+    "pyqrack-cpu>=1.38.2,<1.41 ; sys_platform != 'darwin'",
 ]
 
 [project.optional-dependencies]

src/bloqade/qasm2/emit/main.py

@@ -5,8 +5,11 @@
 from kirin.ir.dialect import Dialect as Dialect
 
 from bloqade.qasm2.parse import ast
+from bloqade.qasm2.dialects.uop import SingleQubitGate, TwoQubitCtrlGate
+from bloqade.qasm2.dialects.expr import GateFunction
 
 from .base import EmitQASM2Base, EmitQASM2Frame
+from ..dialects.core.stmts import Reset, Measure
 
 
 @dataclass
@@ -94,6 +97,24 @@ def emit_if_else(
 
         cond = emit.assert_node(ast.Cmp, frame.get(stmt.cond))
 
+        # NOTE: we need exactly one of those in the then body in order to emit valid QASM2
+        AllowedThenType = SingleQubitGate | TwoQubitCtrlGate | Measure | Reset
+
+        then_stmts = stmt.then_body.blocks[0].stmts
+        uop_stmts = 0
+        for s in then_stmts:
+            if isinstance(s, AllowedThenType):
+                uop_stmts += 1
+                continue
+
+            if isinstance(s, func.Invoke):
+                uop_stmts += isinstance(s.callee.code, GateFunction)
+
+        if uop_stmts != 1:
+            raise interp.InterpreterError(
+                "Cannot lower if-statement: QASM2 only allows exactly one quantum operation in the body."
+            )
+
         with emit.new_frame(stmt) as then_frame:
             then_frame.entries.update(frame.entries)
             emit.emit_block(then_frame, stmt.then_body.blocks[0])

test/qasm2/emit/test_qasm2_emit.py

@@ -1,3 +1,6 @@
+import pytest
+from kirin.interp import InterpreterError
+
 from bloqade import qasm2
 
 
@@ -221,3 +224,68 @@ def para_u():
 U(0.1, 0.2, 0.3) qreg[0];
 """
     )
+
+
+def test_if():
+    @qasm2.extended
+    def non_empty_else():
+        q = qasm2.qreg(1)
+        c = qasm2.creg(1)
+        qasm2.measure(q, c)
+
+        if c[0] == 1:
+            qasm2.x(q[0])
+        else:
+            qasm2.y(q[0])
+
+        return q
+
+    target = qasm2.emit.QASM2()
+
+    with pytest.raises(InterpreterError):
+        target.emit(non_empty_else)
+
+    @qasm2.extended
+    def multiline_then():
+        q = qasm2.qreg(1)
+        c = qasm2.creg(1)
+        qasm2.measure(q, c)
+
+        if c[0] == 1:
+            qasm2.x(q[0])
+            qasm2.y(q[0])
+
+        return q
+
+    target = qasm2.emit.QASM2()
+
+    with pytest.raises(InterpreterError):
+        target.emit(multiline_then)
+
+    @qasm2.extended
+    def valid_if():
+        q = qasm2.qreg(1)
+        c = qasm2.creg(1)
+        qasm2.measure(q, c)
+
+        if c[0] == 0:
+            qasm2.x(q[0])
+
+        return q
+
+    target = qasm2.emit.QASM2()
+    target.emit(valid_if)
+
+    @qasm2.extended
+    def nested_kernel():
+        q = qasm2.qreg(1)
+        c = qasm2.creg(1)
+        qasm2.measure(q, c)
+
+        if c[0] == 0:
+            valid_if()
+
+        return q
+
+    target = qasm2.emit.QASM2()
+    target.emit(nested_kernel)