Force Interpreter VM when VM tracing requested.

Fixes https://github.com/ethereum/cpp-ethereum/issues/2585.

libethereum/State.cpp

@@ -1137,6 +1137,12 @@ bool State::isTrieGood(bool _enforceRefs, bool _requireNoLeftOvers) const
 
 ExecutionResult State::execute(LastHashes const& _lh, Transaction const& _t, Permanence _p, OnOpFunc const& _onOp)
 {
+	auto onOp = _onOp;
+#if ETH_VMTRACE
+	if (isChannelVisible<VMTraceChannel>())
+		onOp = Executive::simpleTrace(); // override tracer
+#endif
+
 #if ETH_PARANOIA
 	paranoia("start of execution.", true);
 	State old(*this);
@@ -1161,16 +1167,11 @@ ExecutionResult State::execute(LastHashes const& _lh, Transaction const& _t, Per
 	ctrace << toHex(e.t().rlp());
 #endif
 	if (!e.execute())
-#if ETH_VMTRACE
 	{
-		if (isChannelVisible<VMTraceChannel>())
-			e.go(e.simpleTrace());
-		else
-			e.go(_onOp);
+		if (onOp)
+			VMFactory::setKind(VMKind::Interpreter); // force interpreter if a trace is wanted
+		e.go(onOp);
 	}
-#else
-		e.go(_onOp);
-#endif
 	e.finalize();
 
 #if ETH_PARANOIA