> db.baz.find()
{ "_id" : ObjectId("582e293f38b7ba11ec92eda8"), "a" : { "b" : 1, "c" : 1 } }
{ "_id" : ObjectId("582e293f38b7ba11ec92eda9"), "a" : { "b" : 1, "c" : 2 } }

// This fails, implying the object literals are not constants:
> db.baz.aggregate([{$bucket: {groupBy: "$a", boundaries: [{b: 1, c: 1}, {b: 1, c: 2}, {b: 2, c: 3}]}}])
assert: command failed: {
	"ok" : 0,
	"errmsg" : "The $bucket 'boundaries' field must be an array of constant values, but found value: { b: 1.0, c: 1.0 }.",
	"code" : 40191,
	"codeName" : "Location40191"
} : aggregate failed
_getErrorWithCode@src/mongo/shell/utils.js:25:13
doassert@src/mongo/shell/assert.js:16:14
assert.commandWorked@src/mongo/shell/assert.js:370:5
DBCollection.prototype.aggregate@src/mongo/shell/collection.js:1319:5
@(shell):1:1

2016-11-17T17:05:06.583-0500 E QUERY    [main] Error: command failed: {
	"ok" : 0,
	"errmsg" : "The $bucket 'boundaries' field must be an array of constant values, but found value: { b: 1.0, c: 1.0 }.",
	"code" : 40191,
	"codeName" : "Location40191"
} : aggregate failed :
_getErrorWithCode@src/mongo/shell/utils.js:25:13
doassert@src/mongo/shell/assert.js:16:14
assert.commandWorked@src/mongo/shell/assert.js:370:5
DBCollection.prototype.aggregate@src/mongo/shell/collection.js:1319:5
@(shell):1:1

// If the objects are wrapped in a $literal, the $bucket succeeds:
> db.baz.aggregate([{$bucket: {groupBy: "$a", boundaries: [{$literal: {b: 1, c: 1}}, {$literal: {b: 1, c: 2}}, {$literal: {b: 2, c: 3}}]}}])
{ "_id" : { "b" : 1, "c" : 1 }, "count" : 1 }
{ "_id" : { "b" : 1, "c" : 2 }, "count" : 1 }