[llvm-branch-commits] [libcxx] release/22.x: [libc++] Fix vector::append_range growing before the capacity is reached (#183264) (PR #183722)

[via llvm-branch-commits]

https://github.com/llvmbot created https://github.com/llvm/llvm-project/pull/183722

Backport d6fcf47a893475f6f3dbcace29093297b7e9f366

Requested by: @philnik777

>From 73e8810482d74deae73552b4a9e93b254db41126 Mon Sep 17 00:00:00 2001
From: Nikolas Klauser <nikolasklauser at berlin.de>
Date: Fri, 27 Feb 2026 11:12:17 +0100
Subject: [PATCH] [libc++] Fix vector::append_range growing before the capacity
 is reached (#183264)

Currently `vector::append_range` grows even when appending a number of
elements that is exactly equal to its spare capacity, which is
guaranteed by the standard to _not_ happen.

Fixes #183256

(cherry picked from commit d6fcf47a893475f6f3dbcace29093297b7e9f366)
---
 libcxx/include/__vector/vector.h                  |  2 +-
 .../vector/vector.modifiers/append_range.pass.cpp | 15 +++++++++++++--
 2 files changed, 14 insertions(+), 3 deletions(-)

diff --git a/libcxx/include/__vector/vector.h b/libcxx/include/__vector/vector.h
index 0151f7cf4180e..4e48b1c2016b1 100644
--- a/libcxx/include/__vector/vector.h
+++ b/libcxx/include/__vector/vector.h
@@ -487,7 +487,7 @@ class vector {
   _LIBCPP_HIDE_FROM_ABI constexpr void append_range(_Range&& __range) {
     if constexpr (ranges::forward_range<_Range> || ranges::sized_range<_Range>) {
       auto __len = ranges::distance(__range);
-      if (__len < __cap_ - __end_) {
+      if (__len <= __cap_ - __end_) {
         __construct_at_end(ranges::begin(__range), ranges::end(__range), __len);
       } else {
         __split_buffer<value_type, allocator_type> __buffer(__recommend(size() + __len), size(), __alloc_);
diff --git a/libcxx/test/std/containers/sequences/vector/vector.modifiers/append_range.pass.cpp b/libcxx/test/std/containers/sequences/vector/vector.modifiers/append_range.pass.cpp
index 3b4bbf642809a..78dd44482b2b1 100644
--- a/libcxx/test/std/containers/sequences/vector/vector.modifiers/append_range.pass.cpp
+++ b/libcxx/test/std/containers/sequences/vector/vector.modifiers/append_range.pass.cpp
@@ -46,14 +46,25 @@ constexpr bool test() {
       assert(std::ranges::equal(v, std::array{1, 2, 3, 4, 5, 6, 7, 8, -1, -2, -3, -4, -5, -6, -7, -8, -9, -10}));
     }
 
-    { // Ensure no reallocation happens.
+    { // Ensure no reallocation happens, and that we don't invalidate iterators-
       int in[]           = {-1, -2, -3, -4, -5, -6, -7, -8, -9, -10};
       std::vector<int> v = {1, 2, 3, 4, 5, 6, 7, 8};
       v.reserve(v.size() + std::ranges::size(in));
-      assert(v.capacity() >= v.size() + std::ranges::size(in));
+
+      auto ptr     = v.data();
+      auto old_cap = v.capacity();
 
       v.append_range(in);
+      assert(v.capacity() == old_cap);
       assert(std::ranges::equal(v, std::array{1, 2, 3, 4, 5, 6, 7, 8, -1, -2, -3, -4, -5, -6, -7, -8, -9, -10}));
+
+      // vector is allowed to overallocate. Make sure it doesn't reallocate until the new capacity is reached.
+      while (v.size() < old_cap) {
+        int in2[] = {0};
+        v.append_range(in2);
+        assert(v.capacity() == old_cap);
+      }
+      assert(v.data() == ptr);
     }
   }