[cmake-developers] Regression caused by compute-default-dialect topic

[Brad King]

On 09/16/2015 03:14 PM, Brad King wrote:
> That eliminates my concern.

This is now in 'master' as:

 Project: Determine default language dialect for the compiler.
 http://cmake.org/gitweb?p=cmake.git;a=commitdiff;h=7235334a

However, I just discovered that it breaks use of toolchain files
that force a compiler and set the version:

 $ cat ~/toolchain.cmake
 include(CMakeForceCompiler)
 CMAKE_FORCE_C_COMPILER(/usr/bin/gcc GNU)
 set(CMAKE_C_COMPILER_VERSION 5.2)

 $ cmake -DCMAKE_TOOLCHAIN_FILE=~/toolchain.cmake ../CMake/Tests/COnly
 CMake Error at /.../share/cmake-3.3/Modules/Compiler/GNU-C.cmake:27 (message):
   CMAKE_C_STANDARD_COMPUTED_DEFAULT should be set for GNU (/usr/bin/gcc)
   version 5.2
 Call Stack (most recent call first):
   /.../share/cmake-3.3/Modules/CMakeCInformation.cmake:33 (include)
   CMakeLists.txt:3 (project)

This is because the language standard default is computed while
determining the compiler id, but that is skipped when using a toolchain
file that forces the compiler.  I think we need to:

* Tolerate the lack of CMAKE_<LANG>_STANDARD_COMPUTED_DEFAULT and fall
  back to our old lookup table.

* Allow toolchain files to set CMAKE_<LANG>_STANDARD_DEFAULT themselves
  just as they can set CMAKE_<LANG>_COMPILER_{ID,VERSION} now.

* Add a RunCMake test that uses incrementally more complex toolchain
  files like the above (ID only, ID+version, ID+version+standard).

-----------------------------------------------------------------------

This may also reveal a larger problem in that compiler features
functionality does not work well with toolchain files that force
the compiler.  Doing so skips the test for a working compiler,
but that also skips ABI and Features detection:

 http://www.cmake.org/gitweb?p=cmake.git;a=blob;f=Modules/CMakeTestCCompiler.cmake;hb=v3.3.2#l73

Asking toolchain files to set the ABI information was not too much,
but the compiler feature detection produces a lot of information
that toolchain files should not have to spell out.  We may need to
consider moving the ABI and Feature checks to somewhere that occurs
even when forcing the compiler with a toolchain file.  That is beyond
the scope of the immediate need for a fix to the above error though.

Thanks,
-Brad