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.
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 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" }
+}
base-commit: f95d3d5de72a1c43e8d529bad3ef59afc3214705
--
2.37.3
