non_type_as_type_argument

The name '{0}' isn't a type, so it can't be used as a type argument.

The analyzer produces this diagnostic when an identifier that isn't a type is used as a type argument.

The following code produces this diagnostic because x is a variable, not a type:

var x = 0;
List<x> xList = [];

Change the type argument to be a type:

var x = 0;
List<int> xList = [];