Under MSVC we were ignoring PYBIND11_CPP_STANDARD and simply not
passing any standard (which makes MSVC default to its C++14 mode).
MSVC 2015u3 added the `/std:c++14` and `/std:c++latest` flags; the
latter, under MSVC 2017, enables some C++17 features (such as
`std::optional` and `std::variant`), so it is something we need to
start supporting under MSVC.
This makes the PYBIND11_CPP_STANDARD cmake variable work under MSVC,
defaulting it to /std:c++14 (matching the default -std=c++14 for
non-MSVC).
It also adds a new appveyor test running under MSVC 2017 with
/std:c++latest, which runs (and passes) the
`std::optional`/`std::variant` tests.
Also updated the documentation to clarify the c++ flags and add show
MSVC flag examples.
This both lets us not bother rechecking LTO flags when cmake reinvokes
itself, and also lets the cmake invoker override to specify custom or
no LTO flags by setting the cache variable with
-DPYBIND11_LTO_CXX_FLAGS= when invoking cmake.
Clang on linux currently fails to run cmake:
$ CC=clang CXX=clang++ cmake ..
...
-- Configuring done
CMake Error at tools/pybind11Tools.cmake:135 (target_compile_options):
Error evaluating generator expression:
$<:-flto>
Expression did not evaluate to a known generator expression
Call Stack (most recent call first):
tests/CMakeLists.txt:68 (pybind11_add_module)
But investigating this led to various other -flto detection problems;
this commit thus overhauls LTO flag detection:
- -flto needs to be passed to the linker as well
- Also compile with -fno-fat-lto-objects under GCC
- Pass the equivalent flags to MSVC
- Enable LTO flags for via generator expressions (for non-debug builds
only), so that multi-config builds (like on Windows) still work
properly. This seems reasonable, however, even on single-config
builds (and simplifies the cmake code a bit).
- clang's lto linker plugins don't accept '-Os', so replace it with
'-O3' when doing a MINSIZEREL build
- Enable trying ThinLTO by default for test suite (only affects clang)
- Match Clang$ rather than ^Clang$ because, for cmake with 3.0+
policies in effect, the compiler ID will be AppleClang on macOS.
Add a BUILD_INTERFACE and a pybind11::pybind11 alias for the interface
library to match the installed target.
Add new cmake tests for add_subdirectory and consolidates the
.cpp and .py files needed for the cmake build tests:
Before:
tests
|-- test_installed_module
| |-- CMakeLists.txt
| |-- main.cpp
| \-- test.py
\-- test_installed_target
|-- CMakeLists.txt
|-- main.cpp
\-- test.py
After:
tests
\-- test_cmake_build
|-- installed_module/CMakeLists.txt
|-- installed_target/CMakeLists.txt
|-- subdirectory_module/CMakeLists.txt
|-- subdirectory_target/CMakeLists.txt
|-- main.cpp
\-- test.py
