Planner can attempt to use invalid index when using plan cache due to plan cache key / MatchExpression sorting inconsistency

(function() { "use strict";   db.c.drop(); db.c.createIndex({a: "2dsphere", b: 1}); db.c.createIndex({b: 1});   db.c.insert({a: {type: "Point", coordinates: [-79.88635540008511, 32.76497530165182]}, b: 10}); db.c.insert({a: [-79.88635540008511, 32.76497530165182], b: 12});   var geoPredicate = { $and: [ {b: {$gte: 6}}, { a: { $within: { $polygon: [ [-79.97488975524868, 32.76497530165182], [-79.88635540008511, 32.76497530165182], [-79.88635540008511, 32.72617288558777], [-79.97488975524868, 32.72617288558777] ] } } }, { a: { $within: { $centerSphere: [[-79.86978928637699, 32.75469246872076], 0.000757002271006813] } } } ] };   // First invocation should succeed and generate a plan cache entry. assert.eq(2, db.c.find(geoPredicate).itcount());   // Second invocation is pulled from the cache and trips the verify()! assert.eq(2, db.c.find(geoPredicate).itcount()); })();