Re: [PATCH] c++: Quash -Wdangling-reference for member operator* [PR107488]

[Jason Merrill <jason@redhat.com>]

On 11/1/22 18:06, Marek Polacek wrote:
> -Wdangling-reference complains here:
> 
>    std::vector<int> v = ...;
>    std::vector<int>::const_iterator it = v.begin();
>    while (it != v.end()) {
>      const int &r = *it++; // warning
>    }
> 
> because it sees a call to
> __gnu_cxx::__normal_iterator<const int*, std::vector<int> >::operator*
> which returns a reference and its argument is a TARGET_EXPR representing
> the result of
> __gnu_cxx::__normal_iterator<const int*, std::vector<int> >::operator++
> But 'r' above refers to one of the int elements of the vector 'v', not
> to a temporary object.  Therefore the warning is a false positive.
> 
> I suppose code like the above is relatively common (the warning broke
> cppunit-1.15.1 and a few other projects), so presumably it makes sense
> to suppress the warning when it comes to member operator*.  In this case
> it's defined as
> 
>        reference
>        operator*() const _GLIBCXX_NOEXCEPT
>        { return *_M_current; }
> 
> and I'm guessing a lot of member operator* are like that, at least when
> it comes to iterators.  I've looked at _Fwd_list_iterator,
> _Fwd_list_const_iterator, __shared_ptr_access, _Deque_iterator,
> istream_iterator, etc, and they're all like that, so adding #pragmas
> would be quite tedious.  :/

> Bootstrapped/regtested on x86_64-pc-linux-gnu, ok for trunk?

OK.

It also occurred to me that we should avoid warning if the reference 
we're initializing is a non-const lvalue reference, which can't bind to 
a temporary.

Maybe also if the function returns a non-const lvalue reference.

> 	PR c++/107488
> 
> gcc/cp/ChangeLog:
> 
> 	* call.cc (do_warn_dangling_reference): Quash -Wdangling-reference
> 	for member operator*.
> 
> gcc/testsuite/ChangeLog:
> 
> 	* g++.dg/warn/Wdangling-reference5.C: New test.
> ---
>   gcc/cp/call.cc                                | 12 +++++++++-
>   .../g++.dg/warn/Wdangling-reference5.C        | 22 +++++++++++++++++++
>   2 files changed, 33 insertions(+), 1 deletion(-)
>   create mode 100644 gcc/testsuite/g++.dg/warn/Wdangling-reference5.C
> 
> diff --git a/gcc/cp/call.cc b/gcc/cp/call.cc
> index c7c7a122045..2c0fa37f53a 100644
> --- a/gcc/cp/call.cc
> +++ b/gcc/cp/call.cc
> @@ -13467,7 +13467,17 @@ do_warn_dangling_reference (tree expr)
>   	       can be e.g.
>   		 const int& z = std::min({1, 2, 3, 4, 5, 6, 7});
>   	       which doesn't dangle: std::min here returns an int.  */
> -	    || !TYPE_REF_OBJ_P (TREE_TYPE (TREE_TYPE (fndecl))))
> +	    || !TYPE_REF_OBJ_P (TREE_TYPE (TREE_TYPE (fndecl)))
> +	    /* Don't emit a false positive for:
> +		std::vector<int> v = ...;
> +		std::vector<int>::const_iterator it = v.begin();
> +		const int &r = *it++;
> +	       because R refers to one of the int elements of V, not to
> +	       a temporary object.  Member operator* may return a reference
> +	       but probably not to one of its arguments.  */
> +	    || (DECL_NONSTATIC_MEMBER_FUNCTION_P (fndecl)
> +		&& DECL_OVERLOADED_OPERATOR_P (fndecl)
> +		&& DECL_OVERLOADED_OPERATOR_IS (fndecl, INDIRECT_REF)))
>   	  return NULL_TREE;
>   
>   	/* Here we're looking to see if any of the arguments is a temporary
> diff --git a/gcc/testsuite/g++.dg/warn/Wdangling-reference5.C b/gcc/testsuite/g++.dg/warn/Wdangling-reference5.C
> new file mode 100644
> index 00000000000..59b5538aee5
> --- /dev/null
> +++ b/gcc/testsuite/g++.dg/warn/Wdangling-reference5.C
> @@ -0,0 +1,22 @@
> +// PR c++/107488
> +// { dg-do compile }
> +// { dg-options "-Wdangling-reference" }
> +
> +#include <vector>
> +
> +int
> +do_sum (std::vector<int>& v)
> +{
> +  int sum = 0;
> +
> +  std::vector<int>::const_iterator it = v.begin();
> +  while (it != v.end())
> +    {
> +      // R refers to one of the int elements of V, not to a temporary
> +      // object, so no dangling reference here.
> +      const int &r = *it++; // { dg-bogus "dangling reference" }
> +      sum += r;
> +    }
> +
> +  return sum;
> +}
> 
> base-commit: 2b0e81d5cc2f7e1d773f6c502bd65b097f392675