[Bug c++/986] g++ misses warning for & on temporary

["driscoll at cs dot wisc dot edu" <gcc-bugzilla at gcc dot gnu dot org>]

------- Comment #26 from driscoll at cs dot wisc dot edu  2010-08-27 22:02 -------
I was surprised to see this is not caught by a warning as well. (The discussion
of whether it should be an error is silly; it pretty clearly shouldn't be.
There's -Werror if you disagree.)

Motivation: This surfaced in the code I'm working on as it has several similar
functions that return std::sets of objects. I changed several of them to return
by const-reference for efficiency purposes, but not all of them could be
modified such. I then changed call sites so that the return was stored in a
const-reference instead of copied. I expected the compiler to be able to catch
any mismatches (storing the by-value return in a reference), but when I ran a
test to see if it would, there was no warning.

Examples:

This first one is inspired by the code I was working on:
  int retByVal() {
      return 5;
  }
  int foo() {
      int const & v = retByVal();  // no warning
      return v;                    // no warning
  }
but you don't even need to get that complicated. The following illustrates as
well:
  int foo() {
      int const & x = 4;  // no warning
      return x;           // no warning
  }


Comparison of compilers:
- GCC fails to warn for either examples:
    - 3.3.3, 3.4.6 (-Wall -W)
    - 4.1.2, 4.5.1 (-Wall -Wextra)
- MSVC fails to warn for either of these examples
    - 2005, 2008 (level 4)
    - 2010 (all warnings, /Wall)
- The EDG front end warns if remarks are on:
    - Intel CC (10.1 -Wall),  *does* warn about these examples. E.g.:
        remark #383: value copied to temporary, reference to temporary used
        int const & v = retByVal();
    - Comeau (with -r) warns


I haven't lost debugging time to this or anything like that, but I am a bit
disappointed and surprised to see that neither MSVC nor GCC warns for this
almost-certain error and thought I'd contribute in the hopes that another
person running across it might inspire someone to fix it.


-- 

driscoll at cs dot wisc dot edu changed:

           What    |Removed                     |Added
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                 CC|                            |driscoll at cs dot wisc dot
                   |                            |edu


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=986