Disabled opcodes are `fail if pc passes' not fail if execute.

Detected by bitcoind negative tests.

opcode.go

@@ -862,6 +862,45 @@ type parsedOpcode struct {
 	opfunc func(op parsedOpcode, s Script) error
 }
 
+// The following opcodes are disabled and are thus always bad to see in the
+// instruction stream (even if turned off by a conditional).
+func (pop *parsedOpcode) disabled() bool {
+	switch pop.opcode.value {
+	case OP_CAT:
+		return true
+	case OP_SUBSTR:
+		return true
+	case OP_LEFT:
+		return true
+	case OP_RIGHT:
+		return true
+	case OP_INVERT:
+		return true
+	case OP_AND:
+		return true
+	case OP_OR:
+		return true
+	case OP_XOR:
+		return true
+	case OP_2MUL:
+		return true
+	case OP_2DIV:
+		return true
+	case OP_MUL:
+		return true
+	case OP_DIV:
+		return true
+	case OP_MOD:
+		return true
+	case OP_LSHIFT:
+		return true
+	case OP_RSHIFT:
+		return true
+	default:
+		return false
+	}
+}
+
 // The following opcodes are always illegal when passed over by the program
 // counter even if in a non-executed branch. (it isn't a coincidence that they
 // are conditionals).

script.go

@@ -603,7 +603,9 @@ func (m *Script) Step() (done bool, err error) {
 	if m.condStack[0] != OpCondTrue {
 		// some opcodes still 'activate' if on the non-executing side
 		// of conditional execution
-		if opcode.alwaysIllegal() {
+		if opcode.disabled() {
+			return true, StackErrOpDisabled
+		} else if opcode.alwaysIllegal() {
 			return true, StackErrAlwaysIllegal
 		} else if opcode.conditional() {
 			executeInstr = true