[gcc/devel/rust/master] c++: Quash -Wdangling-reference for member operator* [PR107488]

[gcc/devel/rust/master] c++: Quash -Wdangling-reference for member operator* [PR107488]

[Thomas Schwinge]

https://gcc.gnu.org/g:5c2e6c93d919b939ba7eca2100fa35cc444f4b8d

commit 5c2e6c93d919b939ba7eca2100fa35cc444f4b8d
Author: Marek Polacek <polacek@redhat.com>
Date:   Tue Nov 1 17:05:52 2022 -0400

    c++: Quash -Wdangling-reference for member operator* [PR107488]
    
    -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.  :/
    
            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.
    
    (cherry picked from commit 32a06ce38a38bf37db468f0e6c83520fcc221534)

Diff:
---
 gcc/cp/call.cc                                   | 12 +++++++++++-
 gcc/testsuite/g++.dg/warn/Wdangling-reference5.C | 22 ++++++++++++++++++++++
 2 files changed, 33 insertions(+), 1 deletion(-)

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;
+}