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

[Patrick Palka <ppalka@redhat.com>]

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.

> 
> 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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