Fix a codegen bug

If we could statically determine that an object doesn't have a __nonzero__ method,
we would previously say that it had an undefined truth value (and then crash).

src/codegen/compvars.cpp

@@ -1578,9 +1578,16 @@ public:
 
     ConcreteCompilerVariable* nonzero(IREmitter& emitter, const OpInfo& info, ConcreteCompilerVariable* var) override {
         static const std::string attr("__nonzero__");
+
+        bool no_attribute = false;
         ConcreteCompilerVariable* called_constant
-            = tryCallattrConstant(emitter, info, var, &attr, true, ArgPassSpec(0, 0, 0, 0), {}, NULL);
-        if (called_constant)
+            = tryCallattrConstant(emitter, info, var, &attr, true, ArgPassSpec(0, 0, 0, 0), {}, NULL, &no_attribute);
+
+        // TODO: if no_attribute, we could optimize by continuing the dispatch process and trying
+        // to call __len__ (and if that doesn't exist, returning a static true).
+        // For now, I'd rather not duplicate the dispatch behavior between here and objmodel.cpp::nonzero.
+
+        if (called_constant && !no_attribute)
             return called_constant;
 
         if (cls == bool_cls) {

test/tests/nonzero_static.py

+
+# In certain cases, we can statically-determine that a variable is truthy.
+# In the past, we would crash in irgen because of this, so for now this is just
+# a crash test.
+
+def f(x):
+    print hasattr(x, "__nonzero__")
+    if x:
+        print "true"
+
+# Objects without nonzero methods that are still true:
+f(file)
+f(open("/dev/null"))
+f(Exception())