[cfe-dev] Semantics for dereferencing incomplete types?

[Neil Booth]

Eli Friedman wrote:-

> > Correct behaviour there depends on dialect:
> >
> > neil at duron:~/src/c$ ~/src/cfe/cfe /tmp/bug.c --c90
> > "/tmp/bug.c", line 4: error: expression must be an lvalue or a function
> >   void* y = &*x;
> >              ^
> > "/tmp/bug.c", line 4: warning: variable "y" declared but not used
> >   void* y = &*x;
> >         ^
> >
> > 1 error found compiling "/tmp/bug.c".
> > neil at duron:~/src/c$ ~/src/cfe/cfe /tmp/bug.c --c99
> > "/tmp/bug.c", line 4: warning: variable "y" declared but not used
> >   void* y = &*x;
> >         ^
> 
> Right... it's actually a completely different case from the first two
> functions because void expressions aren't lvalues.
> 
> Wow, the C standard is really confusing.

It's worse.  qualified void is an lvalue, so qualifying the voids
in C90 should be accepted not rejected :)  So for example (excuse
my posting more output from my FE) in the following c is bad but
d is OK:

$ ~/src/cfe/cfe /tmp/bug.c --c90
"/tmp/bug.c", line 1: error: expression must be an lvalue or a function
void f(void) { void *a; const void *b; void *c = &*a; const void *d =
&*b; }
                                                  ^
"/tmp/bug.c", line 1: warning: variable "c" declared but not used
void f(void) { void *a; const void *b; void *c = &*a; const void *d =
&*b; }
                                             ^
"/tmp/bug.c", line 1: warning: variable "d" declared but not used
void f(void) { void *a; const void *b; void *c = &*a; const void *d =
&*b; }
                                                                  ^

1 error found compiling "/tmp/bug.c".

GCC doesn't get all this stuff right from what I remember; there
are outstanding PRs.  It's not hard to get right though.

Neil.