Re: [PATCH v4] c++: -Wdangling-reference with reference wrapper [PR107532]

[Jason Merrill <jason@redhat.com>]

On 3/2/23 16:24, Marek Polacek wrote:
> On Wed, Mar 01, 2023 at 04:53:23PM -0500, Jason Merrill wrote:
>>> @@ -13791,12 +13830,39 @@ std_pair_ref_ref_p (tree t)
>>>         const int& y = (f(1), 42); // NULL_TREE
>>>         const int& z = f(f(1)); // f(f(1))
>>> -   EXPR is the initializer.  */
>>> +   EXPR is the initializer.  If ARG_P is true, we're processing an argument
>>> +   to a function; the point is to distinguish between, for example,
>>> +
>>> +     Ref::inner (&TARGET_EXPR <D.2839, F::foo (fm)>)
>>> +
>>> +   where we shouldn't warn, and
>>> +
>>> +     Ref::inner (&TARGET_EXPR <D.2908, F::foo (&TARGET_EXPR <...>)>)
>>> +
>>> +   where we should warn (Ref is a reference_like_class_p so we see through
>>> +   it.  */
>>>    static tree
>>> -do_warn_dangling_reference (tree expr)
>>> +do_warn_dangling_reference (tree expr, bool arg_p)
>>>    {
>>>      STRIP_NOPS (expr);
>>> +  if (TREE_CODE (expr) == ADDR_EXPR)
>>> +    expr = TREE_OPERAND (expr, 0);
>>
>> I think if we move this here, we also need to check that expr before
>> STRIP_NOPS had REFERENCE_TYPE.  OK with that change.
> 
> Sorry but I don't think I can do that.  There can be CONVERT_EXPRs
> that need to be stripped, whether arg_p or !arg_p.  For example, we can get
> (const int *) f ((const int &) &TARGET_EXPR <D.2765, NON_LVALUE_EXPR <10>>)
> for
> const int& r5 = (42, f(10));

I meant that we only want to strip ADDR_EXPR if 'expr' at the start of 
the function had REFERENCE_TYPE, corresponding to

>             /* Check that this argument initializes a reference, except for                                                         
>                the argument initializing the object of a member function.  */
>             if (!DECL_NONSTATIC_MEMBER_FUNCTION_P (fndecl)
>                 && !TYPE_REF_P (TREE_TYPE (arg)))
>               continue;

above the code for stripping an ADDR_EXPR from an argument that your 
patch removes.

If the original expr is a pointer rather than a reference, we don't want 
to complain about it pointing to a temporary.

Jason