Clang: For MSVC ABI do not use modes older than C++14

Since commit d44c0db0b2 (clang: setup correct configuration in gnu mode,
2019-02-20, v3.15.0-rc1~41^2~5) we support the GNU-like Clang that
targets the MSVC ABI.  However, Clang cannot compile with the MSVC
standard library unless it runs in a mode aware of C++14 (since MSVC
itself does not even have a lower mode).  When `CMAKE_CXX_STANDARD` is
set to 98 or 11, use C++14 anyway.

Since Clang's default mode is aware of C++14, another option is to not
add any flags for 98 or 11.  However, if a future Clang version ever
defaults to a higher C++ standard, setting the standard to 98 or 11
should at least not use a mode higher than 14.

Also revert test updates from commit 4819ff9647 (Tests: fix failures
with gnu mode clang on windows, 2019-03-21, v3.15.0-rc1~41^2~3) that
were meant to work around the standard selection problem.

Fixes: #19496

1 files changed, 7 insertions, 0 deletions

diff --git a/Modules/Compiler/Clang-CXX.cmake b/Modules/Compiler/Clang-CXX.cmake
index 17f3917..61709f8 100644
--- a/Modules/Compiler/Clang-CXX.cmake
+++ b/Modules/Compiler/Clang-CXX.cmake
@@ -58,6 +58,13 @@ if("x${CMAKE_CXX_COMPILER_FRONTEND_VARIANT}" STREQUAL "xGNU")
   unset(_clang_version_std17)
 
   if("x${CMAKE_CXX_SIMULATE_ID}" STREQUAL "xMSVC")
+    # The MSVC standard library requires C++14, and MSVC itself has no
+    # notion of operating in a mode not aware of at least that standard.
+    set(CMAKE_CXX98_STANDARD_COMPILE_OPTION "-std=c++14")
+    set(CMAKE_CXX98_EXTENSION_COMPILE_OPTION "-std=gnu++14")
+    set(CMAKE_CXX11_STANDARD_COMPILE_OPTION "-std=c++14")
+    set(CMAKE_CXX11_EXTENSION_COMPILE_OPTION "-std=gnu++14")
+
     # This clang++ is missing some features because of MSVC compatibility.
     unset(CMAKE_CXX11_STANDARD__HAS_FULL_SUPPORT)
     unset(CMAKE_CXX14_STANDARD__HAS_FULL_SUPPORT)