This happens with dmd 2.058 and probably earlier ones too. Compile the following with dmd -inline. Note how the assertions fail when the constness of the arguments to the array append doesn't match:

char[] toStr(char[] s)
{
    return s;
}

const(char)[] toStr(const(char)[] s)
{
    return s;
}

void main()
{
    {
        auto str = "abcd";
        const(char)[] res;
        res ~= toStr(str);
        assert(res == str); // Fine
    }
    
    {
        auto str = "abcd".dup;
        char[] res;
        res ~= toStr(str);
        assert(res == str); // Fine
    }
    
    {
        auto str = "abcd";
        char[] res;
        res ~= toStr(str);
        assert(res == str); // Fail
    }
    
    {
        auto str = "abcd".dup;
        const(char)[] res;
        res ~= toStr(str);
        assert(res == str); // Fail
    }
}