[PATCH] c++: -Wdangling-reference and system headers

[Marek Polacek via Gcc-patches]

I got this testcase:

  auto f() -> std::optional;
  for (char c : f().value()) { }

which has a dangling reference: std::optional::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.

Bootstrapped/regtested on x86_64-pc-linux-gnu, ok for trunk?

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.
---
 gcc/cp/call.cc   |  7 +++
 gcc/testsuite/g++.dg/warn/Wdangling-reference4.C | 14 ++
 2 files changed, 21 insertions(+)
 create mode 100644 gcc/testsuite/g++.dg/warn/Wdangling-reference4.C

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 000..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 
+#include 
+
+auto f() -> std::optional;
+
+void
+g ()
+{
+  for (char c : f().value()) { (void) c; } // { dg-warning "dangling 
reference" }
+}

base-commit: f95d3d5de72a1c43e8d529bad3ef59afc3214705
-- 
2.37.3

Re: [PATCH] c++: -Wdangling-reference and system headers

[Jason Merrill via Gcc-patches]

On 10/27/22 11:39, Marek Polacek wrote:

I got this testcase:

   auto f() -> std::optional;
   for (char c : f().value()) { }

which has a dangling reference: std::optional::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.

Bootstrapped/regtested on x86_64-pc-linux-gnu, ok for trunk?

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.
---
  gcc/cp/call.cc   |  7 +++
  gcc/testsuite/g++.dg/warn/Wdangling-reference4.C | 14 ++
  2 files changed, 21 insertions(+)
  create mode 100644 gcc/testsuite/g++.dg/warn/Wdangling-reference4.C

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));


Hmm, this is OK, but maybe we want a 
warning_enabled_at_ignore_system_header?



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 000..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 
+#include 
+
+auto f() -> std::optional;
+
+void
+g ()
+{
+  for (char c : f().value()) { (void) c; } // { dg-warning "dangling 
reference" }
+}

base-commit: f95d3d5de72a1c43e8d529bad3ef59afc3214705