test_composite_key.cpp fails to compile for C++23

[StephanTLavavej]

I work on MSVC's C++ Standard Library implementation, and we regularly build Boost (along with many other open-source projects) to identify compiler/library bugs that would break your project, so we can fix them before shipping. This also allows us to provide advance notice of source-breaking changes - which is the case here.

The paper P2166R1 "Prohibiting basic_string And basic_string_view Construction From nullptr" has been accepted for the upcoming C++23 Standard, and we recently implemented it by merging https://github.com/microsoft/STL/pull/1995 from our contributor @sam20908. Our open-source test pass then discovered the following code in Boost.MultiIndex's tests that is now disallowed by C++23:

https://github.com/boostorg/multi_index/blob/436c60cd09225f195dd8f381ca16402b043b5c3d/test/test_composite_key.cpp#L329

std::string str_=0 now fails to compile with:

error C2440: 'default argument': cannot convert from 'int' to 'std::string'
note: No constructor could take the source type, or constructor overload resolution was ambiguous

(This is because the null pointer constant 0 is convertible to const char* and nullptr_t, so it's a compiler error, although with a less clear message than nullptr would generate.) This code has undefined behavior in C++98 through C++20 (as string's constructor has a precondition that the pointer is non-null); C++23 makes it ill-formed.

There are several ways to fix this code, for example std::string str_="", that are backward-compatible with previous Standards.

[joaquintides]

Hi STL, thanks for the super detailed report! Fixed in develop, will merge to master in time for Boost 1.77, when the merging window opens (July 15th).

[joaquintides]

Merged to master.