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

[Jason Merrill <jason@redhat.com>]

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

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 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