[cfe-commits] r117220 - in /cfe/trunk: lib/Analysis/CFG.cpp test/Sema/warn-unreachable.c

Frits van Bommel fvbommel at gmail.com
Tue Oct 26 01:16:41 PDT 2010

On Tue, Oct 26, 2010 at 4:53 AM, Zhongxing Xu <xuzhongxing at gmail.com> wrote:
> Only statements containing control flow could cause 'Block' to be NULL, e.g.
> DoStmt, WhileStmt, CastStmt. They do occur in the RHS of assignments, but
> they do not occur in the LHS of assignments. So I think it's safe here. Or I
> could miss something?

The following code is accepted by both gcc and clang in c++ mode, even
if -pedantic and/or -std=c++98 is passed:
int* iftrue();
int* iffalse();

void conditional_assign (int cond, int val) {
    (cond ? *iftrue() : *iffalse()) = val;

Neither will compile it in C mode as-is, but both are fine with it if
I put move the dereference to before the opening bracket. (Again, even
with -pedantic and/or (-std=c89 or -std=c99)
Apparently C doesn't preserve the lvalue-ness of the conditional
operator's operands, but C++ does. Not really surprising given that
C++ has an int& type but C doesn't; in C++ their types are int& while
in C they're probably plain old ints.

Anyway, this is an example of code clang (like gcc) compiles without
complaining that has control flow in the LHS of an assignment

I didn't check whether statement expressions are allowed in the LHS,
but that could be another case.

More information about the cfe-commits mailing list