GCC's -Wconversion has some issues. For example, good luck getting gcc to /not/ emit a warning for this code, in C or C++. I have yet to find the appropriate cast to avoid a warning. Clang does not warn for this.

    typedef struct {
        unsigned value : 4;
    } S;

    void foo(S* s, unsigned value) {
        // error: conversion from 'unsigned int' to 'unsigned char:4' may change value
        s->value = value;
    }

I mean, I guess I can see the rationale.. it's just annoying to have to resort to using pragmas to turn off -Wconversion whenever I need to assign to a bitfield.