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From: "redi at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug libstdc++/94831] [10 Regression] vector<float[2]> no longer
 compiles
Date: Tue, 28 Apr 2020 21:39:50 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D94831

--- Comment #2 from Jonathan Wakely <redi at gcc dot gnu.org> ---
(In reply to Jonathan Wakely from comment #0)
> In r277342 I changed std::_Construct for P0784R7, "More constexpr
> containers" and the non-void return type means this no longer compiles:
>=20
> #include <vector>
> std::vector<float[2]> v(1);
>=20
> This was never valid in C++98, and in C++11 to C++17 it's undefined (beca=
use
> the pseudo-destructor call in allocator_traits<std::allocator<T>>::destroy
> is ill-formed for arrays). But for libstdc++ it happened to compile, beca=
use
> we elide the calls to trivial destructors.

We should consider making this change in stage 1, to stop silently accepting
that non-standard code in strict mode:

--- a/libstdc++-v3/include/bits/alloc_traits.h
+++ b/libstdc++-v3/include/bits/alloc_traits.h
@@ -734,6 +734,10 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
     _Destroy(_ForwardIterator __first, _ForwardIterator __last,
             allocator<_Tp>&)
     {
+#if __cplusplus >=3D 201103L && defined __STRICT_ANSI__
+      static_assert(!is_array<_Tp>::value,
+         "arrays are not Erasable from containers using std::allocator");
+#endif
       _Destroy(__first, __last);
     }=