Clang: Do not add '-std=' options when simulating MSVC

The `cl.exe` style command line does not accept the `-std=` options.
Instead behave like MSVC where we don't define standard levels.

Fixes: #16266

Modules/Compiler/Clang-C.cmake

@@ -7,14 +7,30 @@ if(WIN32 OR (APPLE AND NOT appleClangPolicy STREQUAL NEW))
 endif()
 
 if(NOT CMAKE_C_COMPILER_VERSION VERSION_LESS 3.4)
-  set(CMAKE_C90_STANDARD_COMPILE_OPTION "-std=c90")
-  set(CMAKE_C90_EXTENSION_COMPILE_OPTION "-std=gnu90")
+  if(NOT "x${CMAKE_C_SIMULATE_ID}" STREQUAL "xMSVC")
+    set(CMAKE_C90_STANDARD_COMPILE_OPTION "-std=c90")
+    set(CMAKE_C90_EXTENSION_COMPILE_OPTION "-std=gnu90")
 
-  set(CMAKE_C99_STANDARD_COMPILE_OPTION "-std=c99")
-  set(CMAKE_C99_EXTENSION_COMPILE_OPTION "-std=gnu99")
+    set(CMAKE_C99_STANDARD_COMPILE_OPTION "-std=c99")
+    set(CMAKE_C99_EXTENSION_COMPILE_OPTION "-std=gnu99")
 
-  set(CMAKE_C11_STANDARD_COMPILE_OPTION "-std=c11")
-  set(CMAKE_C11_EXTENSION_COMPILE_OPTION "-std=gnu11")
+    set(CMAKE_C11_STANDARD_COMPILE_OPTION "-std=c11")
+    set(CMAKE_C11_EXTENSION_COMPILE_OPTION "-std=gnu11")
+  else()
+    # clang-cl doesn't have any of these
+    set(CMAKE_C90_STANDARD_COMPILE_OPTION "")
+    set(CMAKE_C90_EXTENSION_COMPILE_OPTION "")
+
+    set(CMAKE_C99_STANDARD_COMPILE_OPTION "")
+    set(CMAKE_C99_EXTENSION_COMPILE_OPTION "")
+
+    set(CMAKE_C11_STANDARD_COMPILE_OPTION "")
+    set(CMAKE_C11_EXTENSION_COMPILE_OPTION "")
+  endif()
 endif()
 
-__compiler_check_default_language_standard(C 3.4 99 3.6 11)
+if(NOT "x${CMAKE_C_SIMULATE_ID}" STREQUAL "xMSVC")
+  __compiler_check_default_language_standard(C 3.4 99 3.6 11)
+else()
+  set(CMAKE_C_STANDARD_DEFAULT "")
+endif()

Modules/Compiler/Clang-CXX.cmake

@@ -10,30 +10,46 @@ if(APPLE AND NOT appleClangPolicy STREQUAL NEW)
   return()
 endif()
 
-if(NOT CMAKE_CXX_COMPILER_VERSION VERSION_LESS 2.1)
-  set(CMAKE_CXX98_STANDARD_COMPILE_OPTION "-std=c++98")
-  set(CMAKE_CXX98_EXTENSION_COMPILE_OPTION "-std=gnu++98")
-endif()
+if(NOT "x${CMAKE_CXX_SIMULATE_ID}" STREQUAL "xMSVC")
+  if(NOT CMAKE_CXX_COMPILER_VERSION VERSION_LESS 2.1)
+    set(CMAKE_CXX98_STANDARD_COMPILE_OPTION "-std=c++98")
+    set(CMAKE_CXX98_EXTENSION_COMPILE_OPTION "-std=gnu++98")
+  endif()
 
-if(NOT CMAKE_CXX_COMPILER_VERSION VERSION_LESS 3.1)
-  set(CMAKE_CXX11_STANDARD_COMPILE_OPTION "-std=c++11")
-  set(CMAKE_CXX11_EXTENSION_COMPILE_OPTION "-std=gnu++11")
-elseif(NOT CMAKE_CXX_COMPILER_VERSION VERSION_LESS 2.1)
-  set(CMAKE_CXX11_STANDARD_COMPILE_OPTION "-std=c++0x")
-  set(CMAKE_CXX11_EXTENSION_COMPILE_OPTION "-std=gnu++0x")
-endif()
+  if(NOT CMAKE_CXX_COMPILER_VERSION VERSION_LESS 3.1)
+    set(CMAKE_CXX11_STANDARD_COMPILE_OPTION "-std=c++11")
+    set(CMAKE_CXX11_EXTENSION_COMPILE_OPTION "-std=gnu++11")
+  elseif(NOT CMAKE_CXX_COMPILER_VERSION VERSION_LESS 2.1)
+    set(CMAKE_CXX11_STANDARD_COMPILE_OPTION "-std=c++0x")
+    set(CMAKE_CXX11_EXTENSION_COMPILE_OPTION "-std=gnu++0x")
+  endif()
 
-if(NOT CMAKE_CXX_COMPILER_VERSION VERSION_LESS 3.5)
-  set(CMAKE_CXX14_STANDARD_COMPILE_OPTION "-std=c++14")
-  set(CMAKE_CXX14_EXTENSION_COMPILE_OPTION "-std=gnu++14")
-elseif(NOT CMAKE_CXX_COMPILER_VERSION VERSION_LESS 3.4)
-  set(CMAKE_CXX14_STANDARD_COMPILE_OPTION "-std=c++1y")
-  set(CMAKE_CXX14_EXTENSION_COMPILE_OPTION "-std=gnu++1y")
-endif()
+  if(NOT CMAKE_CXX_COMPILER_VERSION VERSION_LESS 3.5)
+    set(CMAKE_CXX14_STANDARD_COMPILE_OPTION "-std=c++14")
+    set(CMAKE_CXX14_EXTENSION_COMPILE_OPTION "-std=gnu++14")
+  elseif(NOT CMAKE_CXX_COMPILER_VERSION VERSION_LESS 3.4)
+    set(CMAKE_CXX14_STANDARD_COMPILE_OPTION "-std=c++1y")
+    set(CMAKE_CXX14_EXTENSION_COMPILE_OPTION "-std=gnu++1y")
+  endif()
 
-if (NOT CMAKE_CXX_COMPILER_VERSION VERSION_LESS 3.5)
-  set(CMAKE_CXX17_STANDARD_COMPILE_OPTION "-std=c++1z")
-  set(CMAKE_CXX17_EXTENSION_COMPILE_OPTION "-std=gnu++1z")
+  if (NOT CMAKE_CXX_COMPILER_VERSION VERSION_LESS 3.5)
+    set(CMAKE_CXX17_STANDARD_COMPILE_OPTION "-std=c++1z")
+    set(CMAKE_CXX17_EXTENSION_COMPILE_OPTION "-std=gnu++1z")
+  endif()
+else()
+  # clang-cl does not know these options because it behaves like cl.exe
+  set(CMAKE_CXX98_STANDARD_COMPILE_OPTION "")
+  set(CMAKE_CXX98_EXTENSION_COMPILE_OPTION "")
+  set(CMAKE_CXX11_STANDARD_COMPILE_OPTION "")
+  set(CMAKE_CXX11_EXTENSION_COMPILE_OPTION "")
+  set(CMAKE_CXX14_STANDARD_COMPILE_OPTION "")
+  set(CMAKE_CXX14_EXTENSION_COMPILE_OPTION "")
+  set(CMAKE_CXX17_STANDARD_COMPILE_OPTION "")
+  set(CMAKE_CXX17_EXTENSION_COMPILE_OPTION "")
 endif()
 
-__compiler_check_default_language_standard(CXX 2.1 98)
+if(NOT "x${CMAKE_CXX_SIMULATE_ID}" STREQUAL "xMSVC")
+  __compiler_check_default_language_standard(CXX 2.1 98)
+else()
+  set(CMAKE_CXX_STANDARD_DEFAULT "")
+endif()