[gcc r13-3551] c++: -Wdangling-reference and system headers

[Marek Polacek <mpolacek@gcc.gnu.org>]

https://gcc.gnu.org/g:e583c86f49b9ef6991b25308a0ad60de9697f24a

commit r13-3551-ge583c86f49b9ef6991b25308a0ad60de9697f24a
Author: Marek Polacek <polacek@redhat.com>
Date:   Wed Oct 26 21:15:53 2022 -0400

    c++: -Wdangling-reference and system headers
    
    I got this testcase:
    
      auto f() -> std::optional<std::string>;
      for (char c : f().value()) { }
    
    which has a dangling reference: std::optional<T>::value returns
    a reference to the contained value, but here it's the f() temporary.
    We warn, which is great, but only with -Wsystem-headers, because
    the function comes from a system header and warning_enabled_at used
    in do_warn_dangling_reference checks diagnostic_report_warnings_p,
    which in this case returned false so we didn't warn.
    
    Fixed as below.  I could also override dc_warn_system_headers so that
    the warning is enabled in system headers always.  With that, I found one
    issue in libstdc++:
    
    libstdc++-v3/include/bits/fs_path.h:1265:15: warning: possibly dangling reference to a temporary [-Wdangling-reference]
     1265 |         auto& __last = *--end();
          |               ^~~~~~
    
    which looks like a true positive as well.
    
    gcc/cp/ChangeLog:
    
            * call.cc (maybe_warn_dangling_reference): Enable the warning in
            system headers if the decl isn't in a system header.
    
    gcc/testsuite/ChangeLog:
    
            * g++.dg/warn/Wdangling-reference4.C: New test.

Diff:
---
 gcc/cp/call.cc                                   |  7 +++++++
 gcc/testsuite/g++.dg/warn/Wdangling-reference4.C | 14 ++++++++++++++
 2 files changed, 21 insertions(+)

diff --git a/gcc/cp/call.cc b/gcc/cp/call.cc
index 951b9fd2a88..c7c7a122045 100644
--- a/gcc/cp/call.cc
+++ b/gcc/cp/call.cc
@@ -13539,6 +13539,13 @@ maybe_warn_dangling_reference (const_tree decl, tree init)
     return;
   if (!TYPE_REF_P (TREE_TYPE (decl)))
     return;
+  /* Don't suppress the diagnostic just because the call comes from
+     a system header.  If the DECL is not in a system header, or if
+     -Wsystem-headers was provided, warn.  */
+  auto wsh
+    = make_temp_override (global_dc->dc_warn_system_headers,
+			  (!in_system_header_at (DECL_SOURCE_LOCATION (decl))
+			   || global_dc->dc_warn_system_headers));
   if (tree call = do_warn_dangling_reference (init))
     {
       auto_diagnostic_group d;
diff --git a/gcc/testsuite/g++.dg/warn/Wdangling-reference4.C b/gcc/testsuite/g++.dg/warn/Wdangling-reference4.C
new file mode 100644
index 00000000000..aee7a29019b
--- /dev/null
+++ b/gcc/testsuite/g++.dg/warn/Wdangling-reference4.C
@@ -0,0 +1,14 @@
+// { dg-do compile { target c++17 } }
+// { dg-options "-Wdangling-reference" }
+// Check that we warn here even without -Wsystem-headers.
+
+#include <optional>
+#include <string>
+
+auto f() -> std::optional<std::string>;
+
+void
+g ()
+{
+  for (char c : f().value()) { (void) c; } // { dg-warning "dangling reference" }
+}