Using variables with declared types leads to errors (int8, uint8)

[rherrmannr]

Current behavior:

// this works as intendend
var x : stdlib.uint8 = 0;
x = 5 + 3;

// this does not work, but it should
var y : stdlib.int8 = 0;
y = 5 + 3;

Expected behavior: Using simple calculations should work, if the type is definied.