[Bug c++/89722] [8/9 regression] strange warning: type qualifiers ignored on cast result type [-Wignored-qualifiers]
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=89722 Marek Polacek changed: What|Removed |Added Status|NEW |RESOLVED CC||mpolacek at gcc dot gnu.org Resolution|--- |WONTFIX --- Comment #12 from Marek Polacek --- Since this seems to work as designed, I'm going to close it.
[Bug c++/89722] [8/9 regression] strange warning: type qualifiers ignored on cast result type [-Wignored-qualifiers]
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=89722 Richard Biener changed: What|Removed |Added Priority|P3 |P2
[Bug c++/89722] [8/9 regression] strange warning: type qualifiers ignored on cast result type [-Wignored-qualifiers]
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=89722 --- Comment #11 from Jonathan Wakely --- (In reply to Martin Sebor from comment #6) > But the code above doesn't trigger either warning when compiled as C. I > think that suggests that either the manual should be updated to explain the > difference or the two front ends should be made to behave the same way. As C++ is clear that a prvalue of non-class type is cv-unqualified, I assume that's also true in C. But it matters less for C, because (except for extensions like __typeof__) the cv-qualifiers of an rvalue don't really matter. In C++ they affect overloading, template argument deduction, decltype results etc. > a data point, Clang doesn't warn in either C or C++ modes on this code so I > wonder if G++ is being overly pedantic here, especially in the typeof cases. As I noted in PR 80544 comment 0, EDG warns about casts to cv-qualified scalar types, and was my inspiration for our warning.
[Bug c++/89722] [8/9 regression] strange warning: type qualifiers ignored on cast result type [-Wignored-qualifiers]
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=89722 Jonathan Wakely changed: What|Removed |Added Status|UNCONFIRMED |NEW Last reconfirmed||2019-03-15 Ever confirmed|0 |1 --- Comment #10 from Jonathan Wakely --- (In reply to Xavier from comment #9) > We are compiling with -std=gnu++98 so decltype is not available there. But __decltype is. > And the "+ 0" trick does not seem to work correctly. It will cause integer promotion for types with smaller rank than int. > Do you have a solution that works with -std=gnu++98 ? __decltype But I think we want to suppress the warning for (__typeof__(expr))x (we do still want to strip the qualifiers from the result type, just not warn).
[Bug c++/89722] [8/9 regression] strange warning: type qualifiers ignored on cast result type [-Wignored-qualifiers]
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=89722
--- Comment #9 from Xavier ---
We are compiling with -std=gnu++98 so decltype is not available there.
And the "+ 0" trick does not seem to work correctly.
% cat toto.c
#include
int main(void) {
char data[128];
printf("%ju\n", sizeof(typeof(*data)));
printf("%ju\n", sizeof(typeof(*data + 0)));
printf("%ju\n", sizeof(typeof((*data) + 0)));
return 0;
}
% ./a.out
1
4
4
Do you have a solution that works with -std=gnu++98 ?
[Bug c++/89722] [8/9 regression] strange warning: type qualifiers ignored on cast result type [-Wignored-qualifiers]
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=89722 --- Comment #8 from Jonathan Wakely --- (In reply to Martin Sebor from comment #6) > But the code above doesn't trigger either warning when compiled as C. Because I only added it for C++, see r248432 and PR 80544. I don't think I considered cases like this, because I don't think I realised __typeof__ keeps cv-qualifiers. I suppose it would be possible to suppress the warning when the target type being cast to is a typeof expression.
[Bug c++/89722] [8/9 regression] strange warning: type qualifiers ignored on cast result type [-Wignored-qualifiers]
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=89722 Richard Biener changed: What|Removed |Added Target Milestone|--- |8.4 --- Comment #7 from Richard Biener --- So everything works as designed? Then please close as WONTFIX.
[Bug c++/89722] [8/9 regression] strange warning: type qualifiers ignored on cast result type [-Wignored-qualifiers]
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=89722
Martin Sebor changed:
What|Removed |Added
CC||msebor at gcc dot gnu.org
--- Comment #6 from Martin Sebor ---
I agree the macros obscure the problem. Here's a simpler test case:
$ cat t.C && gcc -S -Wall -Wextra -Wignored-qualifiers t.C
int f (void) {
typedef const int ConstInt;
return (ConstInt)1;
}
int g (const long *p) {
return (__typeof__ (*p))1;
}
#if __cplusplus
template
T h (T*) {
return (T)1;
}
int hh (const int *p)
{
return h (p);
}
#endif
t.C: In function ‘int f()’:
t.C:3:20: warning: type qualifiers ignored on cast result type
[-Wignored-qualifiers]
3 | return (ConstInt)1;
|^
t.C: In function ‘int g(const long int*)’:
t.C:7:27: warning: type qualifiers ignored on cast result type
[-Wignored-qualifiers]
7 | return (__typeof__ (*p))1;
| ^
But the code above doesn't trigger either warning when compiled as C. I think
that suggests that either the manual should be updated to explain the
difference or the two front ends should be made to behave the same way. As a
data point, Clang doesn't warn in either C or C++ modes on this code so I
wonder if G++ is being overly pedantic here, especially in the typeof cases.
Btw., while G++ manages to avoid warning on template instantiation above, it
does warn the explicit instantiation below. That seems like a bug to me since
there's no way to avoid the warning if the template comes from some component
the user has no control over. (Clang has the same problem.)
gcc -O2 -S -Wall -Wextra -Wignored-qualifiers t.C
template
T h (T*) {
return (T)1;
}
int hh (const int *p)
{
return h (p);
}
template const long h (const long*);
t.C:11:10: warning: type qualifiers ignored on function return type
[-Wignored-qualifiers]
11 | template const long h (const long*);
| ^
[Bug c++/89722] [8/9 regression] strange warning: type qualifiers ignored on cast result type [-Wignored-qualifiers]
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=89722 --- Comment #5 from Jonathan Wakely --- -Wignored-qualifiers (C and C++ only) Warn if the return type of a function has a type qualifier such as "const".
[Bug c++/89722] [8/9 regression] strange warning: type qualifiers ignored on cast result type [-Wignored-qualifiers]
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=89722 --- Comment #4 from Xavier --- Arf I did not understand this was a const problem. Maybe the warning could be a bit clearer ? :) I confirm that it works fine with typeof(*(bits) + 0). This code is in a header shared between C and C++, and actually our codebase is mostly C, so for now it's probably not worth rewriting all our macros with a C++ version. Thanks for the explanation, and for the workaround !
[Bug c++/89722] [8/9 regression] strange warning: type qualifiers ignored on cast result type [-Wignored-qualifiers]
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=89722
--- Comment #3 from Jonathan Wakely ---
(In reply to Xavier from comment #0)
> This does not seem to occur with gcc, nor with g++ <= 7.
I implemented the warning for GCC 8.
> I could not find a workaround.
Stop using macros?
template
constexpr size_t bitsizeof()
{ return sizeof(T) * CHAR_BIT; }
template
T bitmask_nth(T n)
{ return (T)1 << (n & (bitsizeof() - 1)); }
template
T test_bit(T* bits, unsigned n)
{ return bits[n / bitsizeof()] & bitmask_nth((T)n); }
> Clearly I cannot remove the cast : for shifting 32 bits or more, we need to
> be on a int64.
You can either use std::remove_const_t to get the unqualified
version of the type, or just use decltype as Andrew suggested.
The warning seems correct, typeof(*d->tab) is const int64_t, and casting (const
int64_t)1 is defined by the language to be the same as (int64_t)1 so the const
is ignored.
[Bug c++/89722] [8/9 regression] strange warning: type qualifiers ignored on cast result type [-Wignored-qualifiers]
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=89722 --- Comment #2 from Andrew Pinski --- typeof(*(bits)) Try using decltype instead; typeof is a GNU extension. or use: typeof(*(bits) + 0) Which will remove the const part.
[Bug c++/89722] [8/9 regression] strange warning: type qualifiers ignored on cast result type [-Wignored-qualifiers]
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=89722 Andrew Pinski changed: What|Removed |Added Keywords||diagnostic --- Comment #1 from Andrew Pinski --- typeof(*(bits)) or: typeof(*(bits) + 0) Which will remove the const part. Try using decltype instead; typeof is a GNU extension.
