[gcc r14-11507] libstdc++: Fix -Warray-bounds warning in std::vector::resize [PR114945]

[Jonathan Wakely]

https://gcc.gnu.org/g:6b11aed73f6601de32082a4519022d4de1ce8a8f

commit r14-11507-g6b11aed73f6601de32082a4519022d4de1ce8a8f
Author: Jonathan Wakely <jwakely@redhat.com>
Date:   Mon Mar 31 12:30:44 2025 +0100

    libstdc++: Fix -Warray-bounds warning in std::vector::resize [PR114945]
    
    This is yet another false positive warning fix. This time the compiler
    can't prove that when the vector has sufficient excess capacity to
    append new elements, the pointer to the existing storage is not null.
    
    libstdc++-v3/ChangeLog:
    
            PR libstdc++/114945
            * include/bits/vector.tcc (vector::_M_default_append): Add
            unreachable condition so the compiler knows that _M_finish is
            not null.
            * testsuite/23_containers/vector/capacity/114945.cc: New test.
    
    Reviewed-by: Tomasz Kamiński <tkaminsk@redhat.com>
    
    (cherry picked from commit 844eed3364309bd20cbb7d6793a16b7c6b889ba4)

Diff:
---
 libstdc++-v3/include/bits/vector.tcc               |  3 ++
 .../23_containers/vector/capacity/114945.cc        | 36 ++++++++++++++++++++++
 2 files changed, 39 insertions(+)

diff --git a/libstdc++-v3/include/bits/vector.tcc b/libstdc++-v3/include/bits/vector.tcc
index 0458d560075f..0bd1ae411028 100644
--- a/libstdc++-v3/include/bits/vector.tcc
+++ b/libstdc++-v3/include/bits/vector.tcc
@@ -816,6 +816,9 @@ _GLIBCXX_BEGIN_NAMESPACE_CONTAINER
 
 	  if (__navail >= __n)
 	    {
+	      if (!this->_M_impl._M_finish)
+		__builtin_unreachable();
+
 	      _GLIBCXX_ASAN_ANNOTATE_GROW(__n);
 	      this->_M_impl._M_finish =
 		std::__uninitialized_default_n_a(this->_M_impl._M_finish,
diff --git a/libstdc++-v3/testsuite/23_containers/vector/capacity/114945.cc b/libstdc++-v3/testsuite/23_containers/vector/capacity/114945.cc
new file mode 100644
index 000000000000..daafc59d5a9d
--- /dev/null
+++ b/libstdc++-v3/testsuite/23_containers/vector/capacity/114945.cc
@@ -0,0 +1,36 @@
+// { dg-options "-O2 -Werror=stringop-overflow -Werror=array-bounds" }
+// { dg-do compile { target c++11 } }
+
+// Bug libstdc++/114945
+// Sporadic std::vector::resize() -Wstringop-overflow or -Warray-bounds warning
+
+#include <stdint.h>
+#include <vector>
+template <typename a> struct b {
+  void resize(std::size_t c) { d.resize(c); }
+  template <typename e> void f(a, e);
+  std::vector<char> d;
+};
+#include <regex>
+std::regex g;
+uint64_t h;
+uint32_t i;
+struct s {
+  enum class j : size_t;
+  void k();
+  using l = b<j>;
+  std::vector<l> m;
+};
+enum class s::j : size_t { n };
+void o() { g = ""; }
+void s::k() {
+  l p;
+  auto q = uint32_t(), r = uint32_t();
+  if (h)
+    r = i;
+  b<size_t> t;
+  if (q || r)
+    p.f(j::n, 5);
+  t.resize(4);
+  m.push_back(p);
+}