sim: raise an error when overriding a combinationally-driven signal.

Fixes #557.

amaranth/sim/_base.py

@@ -19,6 +19,7 @@ class BaseSignalState:
     __slots__ = ()
 
     signal = NotImplemented
+    is_comb = NotImplemented
 
     curr = NotImplemented
     next = NotImplemented

amaranth/sim/_pyeval.py

@@ -1,5 +1,6 @@
 from amaranth.hdl._ast import *
 from amaranth.hdl._mem import MemoryData
+from amaranth.hdl._ir import DriverConflict
 
 
 def _eval_matches(test, patterns):
@@ -165,6 +166,8 @@ def _eval_assign_inner(sim, lhs, lhs_start, rhs, rhs_len):
         if lhs_start >= len(lhs):
             return
         slot = sim.get_signal(lhs)
+        if sim.slots[slot].is_comb:
+            raise DriverConflict("Combinationally driven signals cannot be overriden by testbenches")
         value = sim.slots[slot].next
         mask = (1 << lhs_stop) - (1 << lhs_start)
         value &= ~mask

amaranth/sim/_pyrtl.py

@@ -479,6 +479,7 @@ class _FragmentCompiler:
             if domain_name == "comb":
                 for signal in domain_signals:
                     signal_index = self.state.get_signal(signal)
+                    self.state.slots[signal_index].is_comb = True
                     emitter.append(f"next_{signal_index} = {signal.init}")
 
                 inputs = SignalSet()

amaranth/sim/pysim.py

@@ -303,10 +303,11 @@ class _Timeline:
 
 
 class _PySignalState(BaseSignalState):
-    __slots__ = ("signal", "curr", "next", "waiters", "pending")
+    __slots__ = ("signal", "is_comb", "curr", "next", "waiters", "pending")
 
     def __init__(self, signal, pending):
         self.signal = signal
+        self.is_comb = False
         self.pending = pending
         self.waiters = {}
         self.curr = self.next = signal.init