When parsing a $setWindowFields stage, we translate the partitionBy to a $sort stage. But $sort can only sort by a field, not an expression. So, if the partitionBy expression is not a field path, we move the expression to a $set stage.

The translation looks roughly like this:

{$setWindowFields: {partitionBy: {$add: ...}, ...} }
=>
{$set: {tmp: {$add ...} } }
{$sort: {tmp: 1} }
{$_internalSetWindowFields: {partitionBy: "$tmp", ...} }
{$unset: 'tmp'}

But this is incorrect if the input has a field named 'tmp' already. (The actual name we use is not "tmp" but something like "_internal_setWindowFields_partitionKey".)

Instead, we should generate a random fieldname, like a UUID (although maybe we want to string-encode it more efficiently than the usual hexadecimal).