[package] boost/1.76: Fails to build on MacOS due to compiler version restrictions

[rprechelt]

Package and Environment Details (include every applicable attribute)

• Package Name/Version: boost/1.7.6 (and later)
• Operating System+version: MacOS 12.0
• Compiler+version: Apple Clang 13.0.0
• Conan version: conan 1.41.0
• Python version: Python 3.9.47

Conan profile (output of conan profile show default or conan profile show <profile> if custom profile is in use)

[settings]
os=Macos
os_build=Macos
arch=armv8
arch_build=armv8
compiler=apple-clang
compiler.version=13.0
compiler.libcxx=libc++
build_type=Release
[options]
[conf]
[build_requires]
[env]

Steps to reproduce (Include if Applicable)

Boost.Math 1.76/1.77 fails to build on MacOS due to a compiler restriction. I get the following error message when attempting to install via Conan:

boost/1.76.0: Invalid ID: Boost.Math requires a C++11 capable compiler

However, earlier versions of boost (<=1.75) does install successfully. The issue is in the definition of _min_compiler_version_default_cxx11 here. When v1.76 was added to Conan, a line was added that requires a compiler version of 99 on apple-clang which in effect just disables building on apple-clang entirely (at least until the year ~2100) considering we are on apple-clang 13.

The motivation given in the diff here says that disabling the MacOS build on apple-clang was due to C++11 support. However, according to this, apple-clang has supported C++11 since apple-clang 10.0 (which in turn is based off clang 6 which is the minimum version used in the definition of _min_compiler_version_default_cxx11). Indeed, the official Boost release page here lists Apple Clang 9.0/10.0/11.0 as supported compilers.

I'm able to build Boost.Math from source on MacOS without Conan but this (>99) change breaks all of my Conan builds. If I manually edit the ConanFile to build with apple-clang by setting the minimum version to 10.0, and set the compiler C++ version to gnu11/gnu14/gnu17 depending upon the project conanfile, I can successfully build Boost.Math so it isn't a limitation of apple-clang.

Is there some subtle reason why changing the minimum version of apple-clang to 10 won't work (because clearly we can build boost 1.76 on apple-clang)? (cc'ing the original folks who made the change @prince-chrismc @madebr)

[madebr]

The problem is that the default c++ standard of the apple-clang compiler is below 2011.
So you need to explicitly do -s compiler.cppstd=11.

Perhaps we need to change the error message from Boost.Math requires a C++11 capable compiler to
Boost.Math requires cppstd>=2011, current one is lower?

[rprechelt]

I think that's a big step in the right direction!

Right now, I found the error confusing since I know my compiler supports C++11 - something like what you said Boost.Math requires cppstd>=2011. Set compiler.cppstd to be greater than 11. would be good (since it provides a solution to the problem for projects that aren't explicitly setting a C++ standard) but anything would be fine.