[Bug c++/19199] [3.3/3.4/4.0/4.1 Regression] Wrong warning about returning a reference to a temporary

["mmitchel at gcc dot gnu dot org" <gcc-bugzilla at gcc dot gnu dot org>]

------- Additional Comments From mmitchel at gcc dot gnu dot org  2005-03-04 08:13 -------
(Of course, the root cause of this problem is that "fold" is being called before
gimplification, which Nathan and I have sermonized about previously.)

There's interesting interplay between this PR and PR 20280.  In particular,
RTH's proposed transformation in Comment #8 is invalid if A and B are
bit-fields, because the address of a bitfield cannot be taken.

Consider:
  
  struct S { int i : 7; };
  S s;
  extern int &b;
  const int &a = (x ? s.i : b);

This cannot be transformed into:

  int *a = &*((x ? t = &s.i : t = &*b), t);

as per Comment #8.

Indeed fold transforms ((s.i > i)? s.i : i) into MAX_EXPR (s.i, i) in this program:

struct S { int i : 7; };
S s;
extern int &i;
bool b;
void g() {
  ((s.i > i) ? s.i : i) = 3;
}

That happens to work only because of the hack where we allow MAX_EXPR as an
lvalue in an assignment in build_modify_expr if neither argument has
side-effects.  (We recreate the COND_EXPR there.)  This change went in as the
fix for PR 7503.  

Roger, I think you need the same hack where you bind COND_EXPRs to references,
take their addresses, etc.  In fact, you should probably just make real_lvalue_p
accept MAX_EXPRs with side-effect free operands.  And, then, wherever we need
lvalues do the conversion that you presently do in build_modify_expr.  That's
consistent with your "MAX_EXPRs are just the canonical form of certain
COND_EXPRs" logic.

-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=19199