Avoid superfluous warnings when compiling Qt code with ccache / clang - c++

Question

I am having the same problem as this guy.
Compiling with clang and ccache I get this warning everytime it encounters a Q_OBJECT:
warning: explicitly assigning value of variable of type 'int' to itself [-Wself-assign]
This only happens when using ccache, compiling the same code with clang alone works fine.
There seems to be a similar issue with macro expansions where the suggested solution is to set the environment variable
CCACHE_CPP2=yes
Unfortunately, this does not seems to fix my issue, or maybe I'm doing it wrong.
I have tried:
Building from command line with
CCACHE_CPP2=yes ninja
export CCACHE_CPP2=yes
ninja
Building from Qt Creator, adding CCACHE_CPP2 to "Build Environment"
Is there anything else I can do to fix this macro expansion issue? I specifically do not want to disable warnings globally (because that's bad) or locally (because that means wrapping all macros in compiler-specific boilerplate).

Answer 1

Try adding -Wno-self-assign to the CPP flags . It should allow you to disable self-assign errors :
CXXFLAGS= $(CXXFLAGS) -Wno-self-assign
or
CPPFLAGS=$(CPPFLAGS) -Wno-self-assign

Answer 2

Forgive me for not having clang to test this with, but I felt I should help anyway. Expanding on Marek's answer, there's the possibility of placing the pragma inside another macro expansion. It's a very ugly solution, but that way you only need to define the macro once, instead of spawning pragmas all over your code base.
#define WARN int&x = x;
#define NO_WARN _Pragma("GCC diagnostic push") \
_Pragma("GCC diagnostic ignored \"-Wuninitialized\"") \
WARN \
_Pragma("GCC diagnostic pop")
int main(){
NO_WARN
}
As you can see, I tested it with gcc(I have no means of testing with clang right now), but that should work fine in clang by substituting "GCC" with "clang" inside the macro(and using -Wself_assign). Applying to your problem(pseudocode):
#ifdef clang
#define MY_Q_OBJECT _Pragma("clang diagnostic push") \
_Pragma("clang diagnostic ignored \"-Wself-assign\"") \
Q_OBJECT \
_Pragma("clang diagnostic pop")
#else
#define MY_Q_OBJECT Q_OBJECT
#endif
class A{
MY_Q_OBJECT // Unfortunately you still need to replace Q_OBJECT on your classes
}
Another ugly downside is that, at least on gcc, I had to run the preprocessor twice for it to work. Can't tell if the same is necessary for clang.

Answer 3

IMO ignoring this warning globally is not a problem. It warns about dummy code, not about potential logic errors caused by typo. That is why I've voted up #MichaelCMS answer.
But there is a way to disable warning only is some section of code:
#pragma clang diagnostic push
#pragma clang diagnostic ignored "-Wself-assign"
Q_OBJECT
#pragma clang diagnostic pop
This should do the trick (if I didn't mess up the flag name), but I don't like it, to much boiler plate macros.