[gcc r15-9134] libstdc++: Fix -Warray-bounds warning in std::vector::resize [PR114945]

[Jonathan Wakely]

https://gcc.gnu.org/g:844eed3364309bd20cbb7d6793a16b7c6b889ba4

commit r15-9134-g844eed3364309bd20cbb7d6793a16b7c6b889ba4
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>

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 7a92f34ec644..66d73b4cfd7e 100644
--- a/libstdc++-v3/include/bits/vector.tcc
+++ b/libstdc++-v3/include/bits/vector.tcc
@@ -768,6 +768,9 @@ _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);
+}