Re: [PATCH] libstdc++: Fix constexpr basic_string union member [PR113294]

[Jonathan Wakely <jwakely@redhat.com>]

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On Mon, 12 Feb 2024 at 22:17, Patrick Palka <ppalka@redhat.com> wrote:

> On Mon, 12 Feb 2024, Paul Keir wrote:
>
> > A call to `basic_string::clear()` in the std::string move assignment
> operator leads to a constexpr error from an access of inactive union member
> `_M_local_buf` in the added test (`test_move()`). Changing
> `__str._M_local_buf` to `__str._M_use_local_data()` in
> `operator=(basic_string&& __str)` fixes this.
> >
> > Running `make check
> RUNTESTFLAGS="conformance.exp=21_strings/basic_string/*"` on
> x86_64-pc-linux-gnu gave unchanged summary results (# of expected passes
> 570 # of expected failures 4 # of unsupported tests 4).
> >
> > I don't have write access.
> >
> > Signed-off-by: Paul Keir <paul.keir@uws.ac.uk>
>
> LGTM!  A ChangeLog entry is missing from the commit message,
> so here's one that we can use for this commit:
>
>         PR libstdc++/113294
>
> libstdc++-v3/ChangeLog:
>
>         * include/bits/basic_string.h (basic_string::operator=): Use
>         _M_use_local_data() instead of _M_local_buf on the moved-from
>         string.
>         * testsuite/21_strings/basic_string/modifiers/constexpr.cc
>         (test_move): New test.
>

The patch is OK for trunk and gcc-13 with that commit msg. Patrick, could
you push it for Paul?

Thank you both.



> >
> > diff --git a/libstdc++-v3/include/bits/basic_string.h
> b/libstdc++-v3/include/bits/basic_string.h
> > index 43efc99bea9..8a695a494ef 100644
> > --- a/libstdc++-v3/include/bits/basic_string.h
> > +++ b/libstdc++-v3/include/bits/basic_string.h
> > @@ -909,7 +909,7 @@ _GLIBCXX_BEGIN_NAMESPACE_CXX11
> >                 __str._M_capacity(__capacity);
> >               }
> >             else
> > -             __str._M_data(__str._M_local_buf);
> > +             __str._M_data(__str._M_use_local_data());
> >           }
> >         else // Need to do a deep copy
> >           assign(__str);
> > diff --git
> a/libstdc++-v3/testsuite/21_strings/basic_string/modifiers/constexpr.cc
> b/libstdc++-v3/testsuite/21_strings/basic_string/modifiers/constexpr.cc
> > index 0e28a6d4487..ad7548f3236 100644
> > ---
> a/libstdc++-v3/testsuite/21_strings/basic_string/modifiers/constexpr.cc
> > +++
> b/libstdc++-v3/testsuite/21_strings/basic_string/modifiers/constexpr.cc
> > @@ -50,3 +50,16 @@ test_erasure()
> >  }
> >
> >  static_assert( test_erasure() );
> > +
> > +constexpr bool
> > +test_move()
> > +{
> > +  std::string s1;
> > +  std::string s2 = "1234567890123456"; // 16 chars: more than
> _S_local_capacity
> > +  s1 = std::move(s2);
> > +  VERIFY( s1 == "1234567890123456" );
> > +
> > +  return true;
> > +}
> > +
> > +static_assert( test_move() );
> > --
> >
> >
> > Please consider the environment and think before you print.
> >
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