error CS1061: 'Collider' does not contain a definition for 'GameObject' and no accessible extension method 'GameObject' accepting a first argument of type 'Collider' could be found (are you missing a using directive or an assembly reference?) code example

Example: 'NewBehaviourScript' does not contain a definition for 'GameObject' and no accessible extension method 'GameObject' accepting a first argument of type 'NewBehaviourScript' could be found (are you missing a using directive or an assembly reference?)

using System.Collections.Generic;  

// The following declaration generates CS0246.  
using myAliasName = List<int>;

// To avoid the error, fully qualify List.  
using myAliasName2 = System.Collections.Generic.List<int>;