Re: [PATCH] c++: Implement -Wdangling-reference [PR106393]

[Marek Polacek <polacek@redhat.com>]

On Tue, Oct 25, 2022 at 11:39:31AM -0400, Jason Merrill wrote:
> On 10/25/22 09:14, Marek Polacek wrote:
> > On Tue, Oct 25, 2022 at 12:34:50PM +0100, Jonathan Wakely wrote:
> > > On Mon, 24 Oct 2022 at 18:30, Jason Merrill wrote:
> > > > 
> > > > On 10/21/22 19:28, Marek Polacek wrote:
> > > > > When testing a previous version of the patch, there were many FAILs in
> > > > > libstdc++'s 22_locale/; all of them because the warning triggered on
> > > > > 
> > > > >     const test_type& obj = std::use_facet<test_type>(std::locale());
> > > > > 
> > > > > but this code looks valid -- std::use_facet doesn't return a reference
> > > > > to its parameter.  Therefore I added code to suppress the warning when
> > > > > the call is std::use_facet.  Now 22_locale/* pass even with the warning
> > > > > on.  We could exclude more std:: functions like this if desirable.
> > > > 
> > > > Instead of adding special cases in the compiler, let's disable the
> > > > warning around the definition of use_facet (and adjust the compiler as
> > > > needed so that avoids the warning).
> > > 
> > > I assume you mean using #pragma here. If we disable it around the
> > > definition of use_facet, will that disable it for callers of
> > > use_facet, or only within the definition of use_facet itself?
> > 
> > Right, a #pragma will not help, it would only disable the warning
> > within the definition of use_facet itself.
> 
> That's why I mentioned adjusting the compiler, i.e. check warning_enabled_at
> (DECL_SOURCE_LOCATION (fn)

Ah, like with -Wdeprecated-copy.  I think I fixed this:

--- a/libstdc++-v3/include/bits/locale_classes.tcc
+++ b/libstdc++-v3/include/bits/locale_classes.tcc
@@ -127,6 +127,8 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
    *  @return  Reference to facet of type Facet.
    *  @throw  std::bad_cast if @p __loc doesn't contain a facet of type _Facet.
   */
+#pragma GCC diagnostic push
+#pragma GCC diagnostic ignored "-Wdangling-reference"
   template<typename _Facet>
     const _Facet&
     use_facet(const locale& __loc)
@@ -141,6 +143,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
       return static_cast<const _Facet&>(*__facets[__i]);
 #endif
     }
+#pragma GCC diagnostic pop


   // Generic version does nothing.

and then just check warning_enabled_at.  Nice!

Marek