Infer return type from assignment

[ixid via Digitalmars-d-learn]

Is it possible to infer a template's return type from what it's 
assigned to? If not is this a difficult or worthless feature to 
add?

OUT fun(IN, OUT)(IN value) {
    return value.to!OUT;
}
void main() {
    float a = 5.0;
    int b = fun(a);
}