Re: [PATCH v2] c++: avoid -Wdangling-reference for std::span-like classes [PR110358]

[Alex Coplan <alex.coplan@arm.com>]

Hi Marek,

On 30/01/2024 13:15, Marek Polacek wrote:
> On Thu, Jan 25, 2024 at 10:13:10PM -0500, Jason Merrill wrote:
> > On 1/25/24 20:36, Marek Polacek wrote:
> > > Better version:
> > > 
> > > Bootstrapped/regtested on x86_64-pc-linux-gnu, ok for trunk?
> > > 
> > > -- >8 --
> > > Real-world experience shows that -Wdangling-reference triggers for
> > > user-defined std::span-like classes a lot.  We can easily avoid that
> > > by considering classes like
> > > 
> > >      template<typename T>
> > >      struct Span {
> > >        T* data_;
> > >        std::size len_;
> > >      };
> > > 
> > > to be std::span-like, and not warning for them.  Unlike the previous
> > > patch, this one considers a non-union class template that has a pointer
> > > data member and a trivial destructor as std::span-like.
> > > 
> > > 	PR c++/110358
> > > 	PR c++/109640
> > > 
> > > gcc/cp/ChangeLog:
> > > 
> > > 	* call.cc (reference_like_class_p): Don't warn for std::span-like
> > > 	classes.
> > > 
> > > gcc/ChangeLog:
> > > 
> > > 	* doc/invoke.texi: Update -Wdangling-reference description.
> > > 
> > > gcc/testsuite/ChangeLog:
> > > 
> > > 	* g++.dg/warn/Wdangling-reference18.C: New test.
> > > 	* g++.dg/warn/Wdangling-reference19.C: New test.
> > > 	* g++.dg/warn/Wdangling-reference20.C: New test.
> > > ---
> > >   gcc/cp/call.cc                                | 18 ++++++++
> > >   gcc/doc/invoke.texi                           | 14 +++++++
> > >   .../g++.dg/warn/Wdangling-reference18.C       | 24 +++++++++++
> > >   .../g++.dg/warn/Wdangling-reference19.C       | 25 +++++++++++
> > >   .../g++.dg/warn/Wdangling-reference20.C       | 42 +++++++++++++++++++
> > >   5 files changed, 123 insertions(+)
> > >   create mode 100644 gcc/testsuite/g++.dg/warn/Wdangling-reference18.C
> > >   create mode 100644 gcc/testsuite/g++.dg/warn/Wdangling-reference19.C
> > >   create mode 100644 gcc/testsuite/g++.dg/warn/Wdangling-reference20.C
> > > 
> > > diff --git a/gcc/cp/call.cc b/gcc/cp/call.cc
> > > index 9de0d77c423..afd3e1ff024 100644
> > > --- a/gcc/cp/call.cc
> > > +++ b/gcc/cp/call.cc
> > > @@ -14082,6 +14082,24 @@ reference_like_class_p (tree ctype)
> > >   	return true;
> > >       }
> > > +  /* Avoid warning if CTYPE looks like std::span: it's a class template,
> > > +     has a T* member, and a trivial destructor.  For example,
> > > +
> > > +      template<typename T>
> > > +      struct Span {
> > > +	T* data_;
> > > +	std::size len_;
> > > +      };
> > > +
> > > +     is considered std::span-like.  */
> > > +  if (NON_UNION_CLASS_TYPE_P (ctype)
> > > +      && CLASSTYPE_TEMPLATE_INSTANTIATION (ctype)
> > > +      && TYPE_HAS_TRIVIAL_DESTRUCTOR (ctype))
> > > +    for (tree field = next_aggregate_field (TYPE_FIELDS (ctype));
> > > +	 field; field = next_aggregate_field (DECL_CHAIN (field)))
> > > +      if (TYPE_PTR_P (TREE_TYPE (field)))
> > > +	return true;
> > > +
> > >     /* Some classes, such as std::tuple, have the reference member in its
> > >        (non-direct) base class.  */
> > >     if (dfs_walk_once (TYPE_BINFO (ctype), class_has_reference_member_p_r,
> > > diff --git a/gcc/doc/invoke.texi b/gcc/doc/invoke.texi
> > > index 6ec56493e59..e0ff18a86f5 100644
> > > --- a/gcc/doc/invoke.texi
> > > +++ b/gcc/doc/invoke.texi
> > > @@ -3916,6 +3916,20 @@ where @code{std::minmax} returns @code{std::pair<const int&, const int&>}, and
> > >   both references dangle after the end of the full expression that contains
> > >   the call to @code{std::minmax}.
> > > +The warning does not warn for @code{std::span}-like classes.  We consider
> > > +classes of the form:
> > > +
> > > +@smallexample
> > > +template<typename T>
> > > +struct Span @{
> > > +  T* data_;
> > > +  std::size len_;
> > > +@};
> > > +@end smallexample
> > > +
> > > +as @code{std::span}-like; that is, the class is a non-union class template
> > > +that has a pointer data member and a trivial destructor.
> > > +
> > >   This warning is enabled by @option{-Wall}.
> > >   @opindex Wdelete-non-virtual-dtor
> > > diff --git a/gcc/testsuite/g++.dg/warn/Wdangling-reference18.C b/gcc/testsuite/g++.dg/warn/Wdangling-reference18.C
> > > new file mode 100644
> > > index 00000000000..e088c177769
> > > --- /dev/null
> > > +++ b/gcc/testsuite/g++.dg/warn/Wdangling-reference18.C
> > > @@ -0,0 +1,24 @@
> > > +// PR c++/110358
> > > +// { dg-do compile { target c++11 } }
> > > +// { dg-options "-Wdangling-reference" }
> > > +// Don't warn for std::span-like classes.
> > > +
> > > +template <typename T>
> > > +struct Span {
> > > +    T* data_;
> > > +    int len_;
> > > +
> > > +    [[nodiscard]] constexpr auto operator[](int n) const noexcept -> T& { return data_[n]; }
> > > +    [[nodiscard]] constexpr auto front() const noexcept -> T& { return data_[0]; }
> > > +    [[nodiscard]] constexpr auto back() const noexcept -> T& { return data_[len_ - 1]; }
> > > +};
> > > +
> > > +auto get() -> Span<int>;
> > > +
> > > +auto f() -> int {
> > > +    int const& a = get().front(); // { dg-bogus "dangling reference" }
> > > +    int const& b = get().back();  // { dg-bogus "dangling reference" }
> > > +    int const& c = get()[0];      // { dg-bogus "dangling reference" }
> > > +
> > > +    return a + b + c;
> > > +}
> > > diff --git a/gcc/testsuite/g++.dg/warn/Wdangling-reference19.C b/gcc/testsuite/g++.dg/warn/Wdangling-reference19.C
> > > new file mode 100644
> > > index 00000000000..053467d822f
> > > --- /dev/null
> > > +++ b/gcc/testsuite/g++.dg/warn/Wdangling-reference19.C
> > > @@ -0,0 +1,25 @@
> > > +// PR c++/110358
> > > +// { dg-do compile { target c++11 } }
> > > +// { dg-options "-Wdangling-reference" }
> > > +// Like Wdangling-reference18.C but not actually a span-like class.
> > > +
> > > +template <typename T>
> > > +struct Span {
> > > +    T* data_;
> > > +    int len_;
> > > +    ~Span ();
> > > +
> > > +    [[nodiscard]] constexpr auto operator[](int n) const noexcept -> T& { return data_[n]; }
> > > +    [[nodiscard]] constexpr auto front() const noexcept -> T& { return data_[0]; }
> > > +    [[nodiscard]] constexpr auto back() const noexcept -> T& { return data_[len_ - 1]; }
> > > +};
> > > +
> > > +auto get() -> Span<int>;
> > > +
> > > +auto f() -> int {
> > > +    int const& a = get().front(); // { dg-warning "dangling reference" }
> > > +    int const& b = get().back();  // { dg-warning "dangling reference" }
> > > +    int const& c = get()[0];      // { dg-warning "dangling reference" }
> > > +
> > > +    return a + b + c;
> > > +}
> > > diff --git a/gcc/testsuite/g++.dg/warn/Wdangling-reference20.C b/gcc/testsuite/g++.dg/warn/Wdangling-reference20.C
> > > new file mode 100644
> > > index 00000000000..463c7380283
> > > --- /dev/null
> > > +++ b/gcc/testsuite/g++.dg/warn/Wdangling-reference20.C
> > > @@ -0,0 +1,42 @@
> > > +// PR c++/109640
> > > +// { dg-do compile { target c++20 } }
> > > +// { dg-options "-Wdangling-reference" }
> > > +// Don't warn for std::span-like classes.
> > > +
> > > +#include <iterator>
> > > +#include <span>
> > > +
> > > +template <typename T>
> > > +struct MySpan
> > > +{
> > > + MySpan(T* data, std::size_t size) :
> > > +    data_(data),
> > > +    size_(size)
> > > + {}
> > > +
> > > + T& operator[](std::size_t idx) { return data_[idx]; }
> > > +
> > > +private:
> > > +    T* data_;
> > > +    std::size_t size_;
> > > +};
> > > +
> > > +template <typename T, std::size_t n>
> > > +MySpan<T const> make_my_span(T const(&x)[n])
> > > +{
> > > +    return MySpan(std::begin(x), n);
> > > +}
> > > +
> > > +template <typename T, std::size_t n>
> > > +std::span<T const> make_span(T const(&x)[n])
> > > +{
> > > +    return std::span(std::begin(x), n);
> > > +}
> > > +
> > > +int main()
> > > +{
> > > +  int x[10]{};
> > > +  int const& y{make_my_span(x)[0]};
> > > +  int const& y2{make_span(x)[0]};
> > 
> > Let's also test that we do warn if the argument to make_*span is a
> > temporary.  OK with that change, assuming it works as expected.
> 
> We do warn then, I've added
>   using T = int[10];
>   [[maybe_unused]] int const& y3{make_my_span(T{})[0]};
>   [[maybe_unused]] int const& y4{make_span(T{})[0]};
> and get two warnings.  I'll push with that adjusted; thanks.

It looks like this patch breaks bootstrap on aarch64-linux-gnu, I see
the following building aarch64-early-ra.cc in stage2:

/work/builds/bstrap/./prev-gcc/xg++ -B/work/builds/bstrap/./prev-gcc/ -B/work/builds/bstrap/aarch64-unknown-linux-gnu/bin/ -nostdinc++ -B/work/builds/bstrap/prev-aarch64-unknown-linux-gnu/libstdc++-v3/src/.libs -B/work/builds/bstrap/prev-aarch64-unknown-linux-gnu/libstdc++-v3/libsupc++/.libs  -I/work/builds/bstrap/prev-aarch64-unknown-linux-gnu/libstdc++-v3/include/aarch64-unknown-linux-gnu  -I/work/builds/bstrap/prev-aarch64-unknown-linux-gnu/libstdc++-v3/include  -I/home/alecop01/toolchain/src/gcc/libstdc++-v3/libsupc++ -L/work/builds/bstrap/prev-aarch64-unknown-linux-gnu/libstdc++-v3/src/.libs -L/work/builds/bstrap/prev-aarch64-unknown-linux-gnu/libstdc++-v3/libsupc++/.libs  -fno-PIE -c   -g -O2 -fno-checking -gtoggle -DIN_GCC    -fno-exceptions -fno-rtti -fasynchronous-unwind-tables -W -Wall -Wno-narrowing -Wwrite-strings -Wcast-qual -Wmissing-format-attribute -Wconditionally-supported -Woverloaded-virtual -pedantic -Wno-long-long -Wno-variadic-macros -Wno-overlength-strings -Werror -fno-common  -DHAVE_CONFIG_H -fno-PIE -I. -I. -I/home/alecop01/toolchain/src/gcc/gcc -I/home/alecop01/toolchain/src/gcc/gcc/. -I/home/alecop01/toolchain/src/gcc/gcc/../include  -I/home/alecop01/toolchain/src/gcc/gcc/../libcpp/include -I/home/alecop01/toolchain/src/gcc/gcc/../libcody  -I/home/alecop01/toolchain/src/gcc/gcc/../libdecnumber -I/home/alecop01/toolchain/src/gcc/gcc/../libdecnumber/bid -I../libdecnumber -I/home/alecop01/toolchain/src/gcc/gcc/../libbacktrace   -I. -I. -I/home/alecop01/toolchain/src/gcc/gcc -I/home/alecop01/toolchain/src/gcc/gcc/. -I/home/alecop01/toolchain/src/gcc/gcc/../include  -I/home/alecop01/toolchain/src/gcc/gcc/../libcpp/include -I/home/alecop01/toolchain/src/gcc/gcc/../libcody  -I/home/alecop01/toolchain/src/gcc/gcc/../libdecnumber -I/home/alecop01/toolchain/src/gcc/gcc/../libdecnumber/bid -I../libdecnumber -I/home/alecop01/toolchain/src/gcc/gcc/../libbacktrace  \
        /home/alecop01/toolchain/src/gcc/gcc/config/aarch64/aarch64-early-ra.cc
/home/alecop01/toolchain/src/gcc/gcc/config/aarch64/aarch64-early-ra.cc: In member function ‘void {anonymous}::early_ra::record_constraints(rtx_insn*)’:
/home/alecop01/toolchain/src/gcc/gcc/config/aarch64/aarch64-early-ra.cc:1858:66: error: possibly dangling reference to a temporary [-Werror=dangling-reference]
 1858 |         for (auto &allocno : get_allocno_subgroup (op).allocnos ())
      |                                                                  ^
/home/alecop01/toolchain/src/gcc/gcc/config/aarch64/aarch64-early-ra.cc:1858:65: note: the temporary was destroyed at the end of the full expression ‘(({anonymous}::early_ra*)this)->{anonymous}::early_ra::get_allocno_subgroup(op).{anonymous}::early_ra::allocno_subgroup::allocnos()’
 1858 |         for (auto &allocno : get_allocno_subgroup (op).allocnos ())
      |                              ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~^~
cc1plus: all warnings being treated as errors
make[3]: *** [/home/alecop01/toolchain/src/gcc/gcc/config/aarch64/t-aarch64:200: aarch64-early-ra.o] Error 1
make[3]: Leaving directory '/work/builds/bstrap/gcc'
make[2]: *** [Makefile:5096: all-stage2-gcc] Error 2
make[2]: Leaving directory '/work/builds/bstrap'
make[1]: *** [Makefile:25405: stage2-bubble] Error 2
make[1]: Leaving directory '/work/builds/bstrap'
make: *** [Makefile:1100: all] Error 2

Is the patch expected to introduce new warnings?

I'll try to reduce a testcase from this where we don't see the warning
before and we see the warning afterwards.

Thanks,
Alex

> 
> Marek
>