Narrowing conversions in C++0x. Is it just me, or does this sound like a breaking change?

Question

C++0x is going to make the following code and similar code ill-formed, because it requires a so-called narrowing conversion of a double to a int<

Answer 1

Narrowing conversion errors interact badly with implicit integer promotion rules.

I had an error with code which looked like

struct char_t {
    char a;
}

void function(char c, char d) {
    char_t a = { c+d };
}

Which produces an narrowing conversion error (which is correct according to the standard). The reason is that c and d implicitly get promoted to int and the resulting int isn't allowed to be narrowed back to char in an initializer list.

OTOH

void function(char c, char d) {
    char a = c+d;
}

is of course still fine (otherwise all hell would break loose). But surprisingly, even

template
void function() {
    char_t a = { c+d };
}

is ok and compiles without a warning if the sum of c and d is less than CHAR_MAX. I still think this is a defect in C++11, but the people there think otherwise - possibly because it isn't easy to fix without get rid of either implicit integer conversion (which is a relict from the past, when people wrote code like char a=b*c/d and expected it to work even if (b*c) > CHAR_MAX) or narrowing conversion errors (which are possibly a good thing).