libcxx sporadic failure in `std/strings/basic.string/string.cons/copy.pass.cpp`

[StephanTLavavej]

Sporadic failures:

Date	PR	Test Run
2023-08-10	#3913	https://dev.azure.com/vclibs/STL/_build/results?buildId=14819&view=results
2023-08-10	#3947 (CI)	https://dev.azure.com/vclibs/STL/_build/results?buildId=14823&view=results
2023-08-17	#3952 (CI)	https://dev.azure.com/vclibs/STL/_build/results?buildId=14889&view=results
2023-09-12	#4024	https://dev.azure.com/vclibs/STL/_build/results?buildId=15013&view=results

D:\a\_work\1\s\llvm-project\libcxx\test\std\strings\basic.string\string.cons\copy.pass.cpp(56): error C2131: expression did not evaluate to a constant

It's highly curious that a compiler error could be sporadic. This might be related to #3862.

[CaseyCarter]

The test in question:

//===----------------------------------------------------------------------===//
//
// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
// See https://llvm.org/LICENSE.txt for license information.
// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
//
//===----------------------------------------------------------------------===//

// <string>

// basic_string(const basic_string<charT,traits,Allocator>& str); // constexpr since C++20

#include <string>
#include <cassert>

#include "test_macros.h"
#include "test_allocator.h"
#include "min_allocator.h"

template <class S>
TEST_CONSTEXPR_CXX20 void
test(S s1)
{
    S s2 = s1;
    LIBCPP_ASSERT(s2.__invariants());
    assert(s2 == s1);
    assert(s2.capacity() >= s2.size());
    assert(s2.get_allocator() == s1.get_allocator());
}

TEST_CONSTEXPR_CXX20 bool test() {
  {
    typedef test_allocator<char> A;
    typedef std::basic_string<char, std::char_traits<char>, A> S;
    test(S(A(3)));
    test(S("1", A(5)));
    test(S("1234567890123456789012345678901234567890123456789012345678901234567890", A(7)));
  }
#if TEST_STD_VER >= 11
  {
    typedef min_allocator<char> A;
    typedef std::basic_string<char, std::char_traits<char>, A> S;
    test(S(A{}));
    test(S("1", A()));
    test(S("1234567890123456789012345678901234567890123456789012345678901234567890", A()));
  }
#endif

  return true;
}

int main(int, char**)
{
  test();
#if TEST_STD_VER > 17
  static_assert(test()); // <-------- This is line 56.
#endif

  return 0;
}

The error message is effectively telling us "something went wrong somewhere in the test", which isn't terribly useful. The test code looks fine: min_allocator uses a fancy pointer min_pointer with a constexpr explicit conversion from min_pointer<void> to min_pointer<T> that can't possibly be constexpr, but the test doesn't use it.

[StephanTLavavej]

I was able to reproduce this locally by reconstructing the full command line:

for /L %I in (1,1,100) do @cl.EXE %STL%\llvm-project\libcxx\test\std\strings\basic.string\string.cons\copy.pass.cpp /I %STL%\llvm-project\libcxx\test\support /nologo /Od /W4 /w14061 /w14242 /w14265 /w14582 /w14583 /w14587 /w14588 /w14749 /w14841 /w14842 /w15038 /w15214 /w15215 /w15216 /w15217 /w15262 /sdl /WX /D_ENABLE_STL_INTERNAL_CHECK /bigobj /EHsc /MTd /std:c++latest /permissive- /utf-8 /FImsvc_stdlib_force_include.h /wd4643 /D_STL_CALL_ABORT_INSTEAD_OF_INVALID_PARAMETER /analyze:autolog- /Zc:preprocessor /wd6262 -link /MANIFEST:EMBED

This is after set STL=D:\GitHub\STL for my repo root, and building the STL and running set_environment.bat to use it. I had to run this several times so it looks like it happens once in every few hundred runs.

Still not sure what's happening here. Seems like a compiler bug.

[StephanTLavavej]

We don't believe we've seen this happen since 2023-09-12. Possibly #4031 merged on 2023-09-21 changed basic_string's code sufficiently to avoid triggering this, or the VS 2022 17.8 Preview 2 toolset update #4028 merged on 2023-09-14 (or any later update) fixed whatever sporadic compiler bug was involved here.