Bite the bullet on all the indirect timetz and datetimetz tzinfo methods:

make the callers figure out the right tzinfo arguments to pass, instead of
making the callees guess.  The code is uglier this way, but it's less
brittle (when the callee guesses, the caller can get surprised).

Modules/datetimemodule.c

@@ -803,13 +803,15 @@ typedef enum {
  * the "naivety" typedef for details.  If the type is aware, *offset is set
  * to minutes east of UTC (as returned by the tzinfo.utcoffset() method).
  * If the type is offset-naive (or unknown, or error), *offset is set to 0.
+ * tzinfoarg is the argument to pass to the tzinfo.utcoffset() method.
  */
 static naivety
-classify_utcoffset(PyObject *op, int *offset)
+classify_utcoffset(PyObject *op, PyObject *tzinfoarg, int *offset)
 {
 	int none;
 	PyObject *tzinfo;
 
+	assert(tzinfoarg != NULL);
 	*offset = 0;
 	tzinfo = get_tzinfo_member(op);	/* NULL means no tzinfo, not error */
 	if (tzinfo == Py_None)
@@ -819,9 +821,7 @@ classify_utcoffset(PyObject *op, int *offset)
 		return (PyTime_Check(op) || PyDate_Check(op)) ?
 		       OFFSET_NAIVE : OFFSET_UNKNOWN;
 	}
-	*offset = call_utcoffset(tzinfo,
-				 PyTimeTZ_Check(op) ? Py_None : op,
-				 &none);
+	*offset = call_utcoffset(tzinfo, tzinfoarg, &none);
 	if (*offset == -1 && PyErr_Occurred())
 		return OFFSET_ERROR;
 	return none ? OFFSET_NAIVE : OFFSET_AWARE;
@@ -838,17 +838,19 @@ classify_utcoffset(PyObject *op, int *offset)
  */
 static int
 classify_two_utcoffsets(PyObject *o1, int *offset1, naivety *n1,
-			PyObject *o2, int *offset2, naivety *n2)
+			PyObject *tzinfoarg1,
+			PyObject *o2, int *offset2, naivety *n2,
+			PyObject *tzinfoarg2)
 {
 	if (get_tzinfo_member(o1) == get_tzinfo_member(o2)) {
 		*offset1 = *offset2 = 0;
 		*n1 = *n2 = OFFSET_NAIVE;
 	}
 	else {
-		*n1 = classify_utcoffset(o1, offset1);
+		*n1 = classify_utcoffset(o1, tzinfoarg1, offset1);
 		if (*n1 == OFFSET_ERROR)
 			return -1;
-		*n2 = classify_utcoffset(o2, offset2);
+		*n2 = classify_utcoffset(o2, tzinfoarg2, offset2);
 		if (*n2 == OFFSET_ERROR)
 			return -1;
 	}
@@ -3177,7 +3179,9 @@ datetime_richcompare(PyDateTime_DateTime *self, PyObject *other, int op)
 	}
 
 	if (classify_two_utcoffsets((PyObject *)self, &offset1, &n1,
-				     other, &offset2, &n2) < 0)
+				    (PyObject *)self,
+				     other, &offset2, &n2,
+				     other) < 0)
 		return NULL;
 	assert(n1 != OFFSET_UNKNOWN && n2 != OFFSET_UNKNOWN);
  	/* If they're both naive, or both aware and have the same offsets,
@@ -3231,7 +3235,8 @@ datetime_hash(PyDateTime_DateTime *self)
 		int offset;
 		PyObject *temp;
 
-		n = classify_utcoffset((PyObject *)self, &offset);
+		n = classify_utcoffset((PyObject *)self, (PyObject *)self,
+				       &offset);
 		assert(n != OFFSET_UNKNOWN);
 		if (n == OFFSET_ERROR)
 			return -1;
@@ -3690,8 +3695,8 @@ time_richcompare(PyDateTime_Time *self, PyObject *other, int op)
 			     other->ob_type->tp_name);
 		return NULL;
 	}
-	if (classify_two_utcoffsets((PyObject *)self, &offset1, &n1,
-				     other, &offset2, &n2) < 0)
+	if (classify_two_utcoffsets((PyObject *)self, &offset1, &n1, Py_None,
+				     other, &offset2, &n2, Py_None) < 0)
 		return NULL;
 	assert(n1 != OFFSET_UNKNOWN && n2 != OFFSET_UNKNOWN);
 	/* If they're both naive, or both aware and have the same offsets,
@@ -3739,7 +3744,7 @@ time_hash(PyDateTime_Time *self)
 		int offset;
 		PyObject *temp;
 
-		n = classify_utcoffset((PyObject *)self, &offset);
+		n = classify_utcoffset((PyObject *)self, Py_None, &offset);
 		assert(n != OFFSET_UNKNOWN);
 		if (n == OFFSET_ERROR)
 			return -1;
@@ -4628,8 +4633,9 @@ datetimetz_subtract(PyObject *left, PyObject *right)
 			int offset1, offset2;
 			PyDateTime_Delta *delta;
 
-			if (classify_two_utcoffsets(left, &offset1, &n1,
-						    right, &offset2, &n2) < 0)
+			if (classify_two_utcoffsets(left, &offset1, &n1, left,
+						    right, &offset2, &n2,
+						    right) < 0)
 				return NULL;
 			assert(n1 != OFFSET_UNKNOWN && n2 != OFFSET_UNKNOWN);
 			if (n1 != n2) {