I'm not sure the question is clearly worded, but an example will be clearer.
I found out that will not work in Java:
int a = ...;
a = 5.0;
but this will:
int a = ...;
a += 5.0;
I.e., it seems that the = operator is type safe but += isn't. Is there any deep reason for this or is it just another arbitrary decision language designers must take.