Clang-cl: Fix #739 compatibility macros & add to AppVeyor

[Farwaykorse]

As described in #739.
When building on Windows with clang-cl.exe (the compatible replacement for cl.exe), both the macros _MSC_VER and __clang__ are defined.

To get the temporary compatibility fix (#703) working with clang-cl, the check for __clang__ has to come first.

---

Adding compilation tests with AppVeyor:

• Investigate curious test failure in Travis CI
  Build 26: on Linux COMPILER=g++-6 BUILD_TYPE=Release GSL_CXX_STANDARD=14
  Resolves [Travis CI] Test broken size fails with GCC 6.5 #745
• Compiling with the latest clang-cl at AppVeyor
  Using the VS 2017 and the LLVM toolset integration (vs extension available on AppVeyor). This tests the use of clang-cl from VS projects.
• Temporary fix for the lld-linker and vcpkg (version installed on AppVeyor) --> No issue with Ninja
  (link using link.exe or update the installed vcpkg version)
  • Request update of vcpkg with AppVeyor CMake error when linking with Clang-cl (lld-link); requires vcpkg update appveyor/ci#2742
  • Replace the git pull of vcpkg with a fallback to the msvc linker
    Works locally but can't get it working on AppVeyor. Therefore sticking with pulling vcpkg. --> Ninja
  • The git pull command on AppVeyor appears to generate a 'remote exception' bce6ed2 --> Ninja
• Blocking errors
  • "error : cannot use 'try'/'throw' with exceptions disabled" while building Catch
  • -fno-exceptions is not recognised by clang-cl'; results in: -Wunknown-argument 100f2b9
  • No support for -std=c++1z; ...
    clang-cl.exe : error : unknown argument ignored in clang-cl: '-std=c++1z' [-Werror,-Wunknown-argument]
    Fix: When building with clang-cl use the version of CMake installed on AppVeyor (v3.12.2) . 9a625ab
    Is there a reason for using CMake v3.8.0? Yes: Issue CMake version #487 discusses the use of older CMake versions to ensure accessibility.
• Potentially valid warnings
  • -Winvalid-noreturn 2238c47 Enabled by Default
    gsl\include\gsl/gsl_assert(123): error : function declared 'noreturn' should not return [-Werror,-Winvalid-noreturn]
  • -Wnewline-eof 137367b -Weverything
    gsl\tests\multi_span_tests.cpp(1785): error : no newline at end of file [-Werror,-Wnewline-eof]
  • -Wunused-member-function 337a03c -Weverything
    gsl\tests\span_tests.cpp(57): error : unused member function 'operator&' [-Werror,-Wunused-member-function]
  • -Wdouble-promotion f810ce7 -Weverything
    gsl\tests\notnull_tests.cpp(196): error : implicit conversion increases floating-point precision: 'float' to 'double' [-Werror,-Wdouble-promotion]
  • -Wmissing-noreturn 1b15a73 -Weverything
    gsl\tests\no_exception_ensure_tests.cpp(29): error : function 'test_terminate' could be declared with attribute 'noreturn' [-Werror,-Wmissing-noreturn]
gsl\tests\no_exception_throw_tests.cpp(28): error : function 'test_terminate' could be declared with attribute 'noreturn' [-Werror,-Wmissing-noreturn]
  • -Wmissing-variable-declarations 329da83 -Weverything
    gsl\tests\utils_tests.cpp(83): error : no previous extern declaration for non-static variable 'j' [-Werror,-Wmissing-variable-declarations]
• Update README.md: supported platform Clang-cl 7.0.0 on Windows. 07ec87b

Extra:

• Replace MSBuild with Add Ninja-build. AppVeyor execution time reduction with 60%.
  • Manually add target architecture flags to clang-cl 449eec4
    Cmake issue: Ninja generator fails with clang-cl when mixing 64/32 bit clang and Visual Studio Environment
  • No need for the lld-link workaround when using Ninja. 76375f1

Improvements: (After review by annagrin)

• Separate CMake generation from build. 4631e6e
• "Clearer"/less convoluted compile flag configuration for the tests. caa29d0
  + 'flatter' decision structure; - some duplication; + more specific to each compiler
  • Remove redundant warning flags for clang-cl (checked documentation since v4.0.0)
    Note: /W4 enables -Wall and -Wextra. 2bdb8f9 767f34c 595a726
  • Remove flag for clang-cl -fno-strict-aliasing no issues building without it. b1e63fd
• Move suppression of -Wunused-member-function from span_tests.cpp to configuration.
  note: With -Weverything add: $<$<EQUAL:${GSL_CXX_STANDARD},14>:-Wno-unused-member-function> c98c546
• Why those additional -Wno-* flags? - Local issue. Remove flags. 7b85156 - pushed warning level.
  • Check changes. (marked in this checklist)
• Different solution for [[noreturn]] in 2238c47
• Find first working CMake version. Proper flags for clang-cl. v7.0.0 does not recognize -std=c++1z.
  Both CMake and clang on AppVeyor are updated regularly. Unless both can be pinned, similar issues will appear again. Besides, this exercises the usage of the latest CMake release.
• Return MSBuild generators. 2197612
• Push warning level.
  • LLVM/clang-cl: -Weverything 9ef9808
    • -Wc++98-compat and -Wc++98-compat-pedantic 57ac640, bef30b8
    • -Wmissing-prototypes in tests f96eca3
    • -Wundef on macro __GNUC__ a987075
  • LLVM/clang++ (Linux & OSX): -Weverything
    • -Wweak-vtables - suppressed fcbbe1f
    • -Wundef on macro _MSC_VER eb60986
    • -Wpadded in gsl_util:51 and multi_span:1231 - suppressed fe882ce
    • -Wundefined-func-template only in clang v5.0.2. e7812cb
    • -Wdeprecated solved e7812cb

      definition of implicit copy assignment operator for 'static_bounds' is deprecated because it has a user-declared copy constructor

• Clang-cl compilation with VS2015 (using the v140 MS STL). f6f7579
  • -Wkeyword-macro Clang does not require the constexpr disabling macro. f40a69e

Resolves #739

[annagrin]

@Farwaykorse thanks for the fix, can we also add test cases in appveyor.yml to make this always true?

[Farwaykorse]

Compiling with clang-cl in Appveyor seems possible, but I don't have any experience with this.

[Farwaykorse]

I've made an error while reordering the options.
I'll wait for Appveyor to finish the last run, to see if there are more issues, before committing.

[Farwaykorse]

Issue with lld-link and the vcpkg install:
Should be fixed when Appveyor updates their vcpkg version. (Requires: >"2018.10.20".)

• llvm toolchain - vcpkg integration - lld-linker error  vcpkg#4359
• https://bugs.llvm.org/show_bug.cgi?id=38985

[Farwaykorse]

Changes:

• The warning level with LLVM/clang is now pushed to -Weverything.
  • No issues with clang-cl on Windows.
  • Compilation with clang++ on Linux and OSX.
    • -Wdeprecated solved, please review. e7812cb

      GSL/include/gsl/multi_span:598:15: warning: definition of implicit copy assignment operator for 'static_bounds<-1>' is deprecated because it has a user-declared copy constructor [-Wdeprecated]
          constexpr static_bounds(const static_bounds&) noexcept = default;
                    ^
      

    • -Wpadded currently suppressed, might be solvable.

      gsl_util:51:7: warning: padding size of 'gsl::final_action<std::_Bind<void (*(std::reference_wrapper<int>))(int &)> >' with 7 bytes to alignment boundary [-Wpadded]
      multi_span:1231:7: error: padding size of 'gsl::multi_span<const int, 2>' with 7 bytes to alignment boundary [-Werror,-Wpadded]
      

    • -Wweak-vtables currently suppressed, might be solvable.

      gsl_assert:91:8: error: 'fail_fast' has no out-of-line virtual method definitions; its vtable will be emitted in every translation unit [-Werror,-Wweak-vtables]
      gsl_util:97:8: error: 'narrowing_error' has no out-of-line virtual method definitions; its vtable will be emitted in every translation unit [-Werror,-Wweak-vtables]
      

• The AppVeyor script now supports both MSBuild and Ninja-build.
  The current setup uses MSBuild by default and Ninja for two cases:
  • VS2017: There is a bug in the LLVM integration extension on AppVeyor.
  • VS2015: MSBuild is not supported, since LLVM v7.0.0.
• Add clang-cl compilation with VS2015 (using the v140 MS STL).

[annagrin]

@Farwaykorse I will resolve conflicts caused by my changes. Thanks @Farwaykorse!

[annagrin]

added PR #762 to resolve conflicts

[Farwaykorse]

Thanks @annagrin and @CaseyCarter for your friendly and constructive feedback.