Change Binder variable type if value is assigned to null variable, or null is assigned to a variable type

Upsilon/Binder/Binder.cs

@@ -159,10 +159,18 @@ namespace Upsilon.Binder
             }
             else
             {
-                if (boundExpression.Type != variable.Type)
+                // don't allow assigning different typed variables to a variable, unless either of them is nil, allow assigning nil to all variables
+                if (boundExpression.Type != variable.Type && (variable.Type != Type.Nil || boundExpression.Type == Type.Nil))
                 {
-                    _diagnostics.LogCannotConvert(boundExpression.Type, variable.Type, e.Span);
-                    return new BoundExpressionStatement(boundExpression);
+                    if (variable.Type == Type.Nil || boundExpression.Type == Type.Nil)
+                    {
+                        variable.Type = boundExpression.Type;
+                    }
+                    else
+                    {
+                        _diagnostics.LogCannotConvert(boundExpression.Type, variable.Type, e.Span);
+                        return new BoundExpressionStatement(boundExpression);
+                    }
                 }
             }
 

Upsilon/Binder/VariableSymbol.cs

@@ -10,7 +10,7 @@ namespace Upsilon.Binder
             Name = name;
         }
 
-        public Type Type { get; }
+        public Type Type { get; set; }
         public string Name { get; }
     }
 }