https://gcc.gnu.org/bugzilla/show_bug.cgi?id=93589
--- Comment #9 from John Downing <jrdowning at yahoo dot com> ---
(In reply to Jason Merrill from comment #7)
> Rather, it was suppressed for the simple case where the RHS has a suitable
> type. As Jonathan says, (CHAR_BIT * static_cast<short>(i)) has type int (it
> would be unsigned without the cast). And then the << expression has type
> int. And then the | expression has type int.
>
> Before the patch for 40752, we would warn based just on the type of the |
> expression. Now we also look at the type of the << expression, see that it
> is also int, and warn. You can avoid the warning by breaking up the
> expression more:
>
> #define CHAR_BIT 8
> void print_byte_order( short )
> {
> short val = 0;
> unsigned i = 1;
> short pat1 = (CHAR_BIT * static_cast<short>(i));
> short pat2 = pat1 << (CHAR_BIT * static_cast<short>(i));
> val |= pat2;
> }
>
> or adding another cast as in comment #1.
The code I posted has two casts, as in comment #1. When I compile code like
this:
#define CHAR_BIT 8
void print_byte_order_short( short )
{
short val = 0;
unsigned int i =1;
for(i = 1; i < sizeof(short); ++i)
{
short part1 = (CHAR_BIT * static_cast<short>(i));
short part2 = part1 << CHAR_BIT * static_cast<short>(i);
val |= part2;
}
cout << val;
}
I get this warning:
example2.cpp: In function void print_byte_order_short(short int):
example2.cpp:15:32: warning: conversion from int to short int may change value
[-Wconversion]
short part1 = (CHAR_BIT * static_cast<short>(i));
~~~~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~
example2.cpp:16:27: warning: conversion from int to short int may change value
[-Wconversion]
short part2 = part1 << CHAR_BIT * static_cast<short>(i);
~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Which seems like the same "design decision" I referenced in comment #8.
