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

[Jonathan Wakely <jwakely@redhat.com>]

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On Sat, 22 Oct 2022 at 00:28, Marek Polacek wrote:
> I've run the testsuite/bootstrap with the warning enabled by default.
> There were just a few FAILs:
> * g++.dg/warn/Wdangling-pointer-2.C
> * 20_util/any/misc/any_cast.cc
> * 20_util/forward/c_neg.cc
> * 20_util/forward/f_neg.cc

These two are XFAIL tests, they're checking for a required static
assert due to misusing std::forward, so I don't think we want to
change them to remove a "bug" (they're meant to be buggy).
I think they should use -Wno-dangling-reference

> * experimental/any/misc/any_cast.cc
> all of these look like genuine bugs.  A bootstrap with the warning
> enabled by default passed.


I've attached a patch for the two any_cast.cc tests that avoids
creating dangling references, without changing the intended behaviour
of the tests.

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diff --git a/libstdc++-v3/testsuite/20_util/any/misc/any_cast.cc b/libstdc++-v3/testsuite/20_util/any/misc/any_cast.cc
index 8d63dfbba9b..43f90c336c9 100644
--- a/libstdc++-v3/testsuite/20_util/any/misc/any_cast.cc
+++ b/libstdc++-v3/testsuite/20_util/any/misc/any_cast.cc
@@ -93,7 +93,8 @@ void test03()
   MoveEnabled m;
   MoveEnabled m2 = any_cast<MoveEnabled>(any(m));
   VERIFY(move_count == 1);
-  MoveEnabled&& m3 = any_cast<MoveEnabled&&>(any(m));
+  auto rvalue = [](MoveEnabled&&) { };
+  rvalue(any_cast<MoveEnabled&&>(any(m))); // No move construction, just a cast.
   VERIFY(move_count == 1);
 }
 
diff --git a/libstdc++-v3/testsuite/experimental/any/misc/any_cast.cc b/libstdc++-v3/testsuite/experimental/any/misc/any_cast.cc
index f3ce83ad702..98347f6d37d 100644
--- a/libstdc++-v3/testsuite/experimental/any/misc/any_cast.cc
+++ b/libstdc++-v3/testsuite/experimental/any/misc/any_cast.cc
@@ -92,7 +92,8 @@ void test03()
   MoveEnabled m;
   MoveEnabled m2 = any_cast<MoveEnabled>(any(m));
   VERIFY(move_count == 1);
-  MoveEnabled&& m3 = any_cast<MoveEnabled&&>(any(m));
+  auto rvalue = [](MoveEnabled&&) { }; // Can only be called with an rvalue.
+  rvalue(any_cast<MoveEnabled&&>(any(m))); // Doesn't move, just casts to rval.
   VERIFY(move_count == 1);
   struct MoveDeleted
   {
@@ -101,8 +102,9 @@ void test03()
     MoveDeleted(const MoveDeleted&) = default;
   };
   MoveDeleted md;
-  MoveDeleted&& md2 = any_cast<MoveDeleted>(any(std::move(md)));
-  MoveDeleted&& md3 = any_cast<MoveDeleted&&>(any(std::move(md)));
+  auto rvalue2 = [](MoveDeleted&&) { }; // Can only be called with an rvalue.
+  rvalue2(any_cast<MoveDeleted>(any(std::move(md))));
+  rvalue2(any_cast<MoveDeleted&&>(any(std::move(md))));
 }
 
 void test05()