NAME
UpvarExistingOrDie — Do upvar if variable exists, else generate error.
ERRORS
Error Conditions
ConditionError CodeError Message
Input variable not found in caller. -1 Could not find variable $VarName in caller.
Local name for variable is empty string. -10 Second argument is missing. Got empty string.
Got empty string instead of variable name. -3 Variable name is missing. Got empty string.
For the main part of the command documentation, see here.