[cfe-dev] -Wunused-private-field distracted by side effects

[Stephan Bergmann]

BaseAndFieldInfo::addFieldInitializer (lib/Sema/SemaDeclCXX.cpp) contains

>     // Check whether this initializer makes the field "used".
>     if (Init->getInit()->HasSideEffects(S.Context))
>       S.UnusedPrivateFields.remove(Init->getAnyMember());

which filters out of the list of unused private member variables those 
with initializers that potentially incur side effects.  That is, in

> int f();
> class C {
>     int i, j;
> public:
>     C(): i(f()), j(1) {}
> };

clang++ -Wall will emit a -Wunused-private-field for j but not for i.

Is that behavior intended?  (Member variables of class type, where the 
constructor itself may have side effects, are already exempted from the 
UnusedPrivateFields list via the InitializationHasSideEffects() check 
elsewhere in lib/Sema/SemaDeclCXX.cpp.)

Of course, such a warning cannot in general be addressed by naively 
removing the member variable along with any initializers, but that is no 
different to e.g. -Wunused-variable, which does get emitted for

> int f();
> void g() {
>     int i(f());
> }

One arcane legitimate reason to not emit a -Wunused-private-field could 
be a case where f() needs to be called for side effects between 
construction of two other members, as in

> int f();
> struct S1 { /*...*/ };
> struct S2 { /*...*/ };
> class C {
>     S1 s1;
>     int dummy;
>     S2 s2;
> public:
>     C(): s1(), dummy(f()), s2() {}
> };

but that could arguably be required to use a  __attribute__((unused)).

Stephan