Unsigned integer axis maps below-range values to overflow
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Assessment
- Difficulty
- 2/5
- Estimated time
- 1-3 hours
- Newbie friendliness
- 76/100
Research direction
Read include/boost/histogram/axis/integer.hpp around line 109 and the Axis index contract in doc/concepts/Axis.qbk. Start by running the supplied repro.cpp with the pinned headers and C++17 build commands; then inspect the unsigned subtraction that produces the reported index. Done means the below-range input returns -1 and increments underflow, while the listed in-range and above-range indices remain as expected.
Written by the indexing model from the issue text.
Description
axis::integer<unsigned>(1, 4) maps 0u to overflow. The Axis contract requires underflow for this below-range value.
Reproduced on x86-64 Ubuntu 24.04 with GCC 13.3.0 (Ubuntu 13.3.0-6ubuntu2~24.04.1) and Clang 18.1.3 (Ubuntu 1ubuntu1). The examples use C++17 Release builds with -O2, without fast-math. Supplemental verifier controls also used -DNDEBUG.
Tested Boost.Histogram commit: 22740e5082ca99d5717da8d9c31d684be9aaeecf. Dependency headers came from Boost superproject commit afa7af297165ad85978b364eb9df19e098109d9e, whose Histogram gitlink matches that pin.
Master still equals the tested commit as checked on 2026-10-12. Default develop at f9017f94684583eeef80eb230768c932a8a6e76c retains the cited expressions. Develop was read, not tested.
From a fresh directory, prepare the pinned sources:
git clone https://github.com/boostorg/histogram.git histogram
git -C histogram checkout --detach 22740e5082ca99d5717da8d9c31d684be9aaeecf
git clone --no-checkout https://github.com/boostorg/boost.git boost
git -C boost checkout --detach afa7af297165ad85978b364eb9df19e098109d9e
git -C boost submodule update --init --jobs 3 \
libs/config libs/core libs/mp11 libs/throw_exception libs/variant2 libs/assert
Save this example as repro.cpp:
#include <boost/histogram.hpp>
#include <iostream>
int main() {
namespace bh = boost::histogram;
auto h = bh::make_histogram(bh::axis::integer<unsigned>(1, 4));
h(0u);
const auto index = h.axis().index(0u);
std::cout << "index(0)=" << index << " underflow=" << h.at(-1)
<< " overflow=" << h.at(3) << '\n';
return index == -1 && h.at(-1) == 1 && h.at(3) == 0 ? 0 : 1;
}
Run these commands in Bash with g++ and clang++-18 on PATH:
includes=(-I"$PWD/histogram/include")
for module in config core mp11 throw_exception variant2 assert; do
includes+=(-I"$PWD/boost/libs/$module/include")
done
for cxx in g++ clang++-18; do
"$cxx" -std=c++17 -O2 -Wall -Wextra "${includes[@]}" \
repro.cpp -o "repro-$cxx"
"./repro-$cxx"
printf 'exit=%s\n' "$?"
done
Both compiler runs print the observed result below and exit 1 because the expected-value check fails.
The Axis index contract assigns -1 to values below the covered range and size() to values above it. The integer axis interface permits integral value types and defines the interval as [start, stop).
Expected for the single fill: index(0u) == -1, underflow 1, overflow 0, and all three ordinary bins 0. Observed:
index(0)=3 underflow=0 overflow=1
For inputs 0u, 1u, 2u, 3u, 4u, 5u, expected indices are -1,0,1,2,3,3. Observed indices are 3,0,1,2,3,3. The independent Python integer oracle also gives flow-inclusive counts 1,1,1,1,2, compared with observed 0,1,1,1,3.
At integer.hpp:109, x - min_ is evaluated in the unsigned value type before conversion to double. For 0u - 1u, the defined unsigned wrap produces a large positive value that selects overflow.
Issue 437, item 7, discusses extreme signed arithmetic at this same source line. The example here uses small unsigned values.
The verifier checked all values from 0 through 5 on the shifted unsigned axis. A signed axis on [1,4) and an unsigned axis on [0,4) give the required indices and counts. These checks cover the small programs and listed controls. The full library test suite, other compilers, other platforms, and C++14/C++20 builds were not run.
The reproduction example is provided under CC0.
Found by ChipLab's AI verification agents and independently reproduced.
ChipLab ref: CLAB-2026-929
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