MSVC: Document and test behavior of empty MSVC_RUNTIME_LIBRARY

Extend tests added by commit fb3370b6a1 (MSVC: Add abstraction for
runtime library selection, 2019-04-10) to cover an empty value for
the property.  It should result in no specific setting.

Issue: #19108

Help/prop_tgt/MSVC_RUNTIME_LIBRARY-VALUES.txt

@@ -13,3 +13,8 @@
 
 The value is ignored on non-MSVC compilers but an unsupported value will
 be rejected as an error when using a compiler targeting the MSVC ABI.
+
+The value may also be the empty string (``""``) in which case no runtime
+library selection flag will be added explicitly by CMake.  Note that with
+:ref:`Visual Studio Generators` the native build system may choose to
+add its own default runtime library selection flag.

Tests/MSVCRuntimeLibrary/CMakeLists.txt

@@ -42,7 +42,22 @@ endfunction()
 function(verify lang src)
   add_library(default-${lang} ${src})
   target_compile_definitions(default-${lang} PRIVATE VERIFY_MT VERIFY_DLL "$<$<CONFIG:Debug>:VERIFY_DEBUG>")
+
   verify_combinations(MultiThreaded ${lang} ${src})
+
+  # Test known MSVC default behavior when no flag is given.
+  if(CMAKE_${lang}_COMPILER_ID STREQUAL "MSVC")
+    set(CMAKE_MSVC_RUNTIME_LIBRARY "")
+    add_library(empty-${lang} ${src})
+    if(CMAKE_${lang}_COMPILER_VERSION VERSION_GREATER_EQUAL 14)
+      # VS 2005 and above default to multi-threaded.
+      target_compile_definitions(empty-${lang} PRIVATE VERIFY_MT)
+    endif()
+    if(CMAKE_GENERATOR MATCHES "Visual Studio ([^9]|9[0-9])")
+      # VS 2010 and above have a different default runtime library for projects than 'cl'.
+      target_compile_definitions(empty-${lang} PRIVATE VERIFY_DLL)
+    endif()
+  endif()
 endfunction()
 
 verify(C verify.c)

Tests/RunCMake/VS10Project/RuntimeLibrary-check.cmake

@@ -20,7 +20,7 @@ macro(RuntimeLibrary_check tgt rtl_expect)
     endif()
   endforeach()
 
-  if(NOT HAVE_Runtimelibrary)
+  if(NOT HAVE_Runtimelibrary AND NOT "${rtl_expect}" STREQUAL "")
     set(RunCMake_TEST_FAILED "Project file ${tgt}.vcxproj does not have a RuntimeLibrary field.")
     return()
   endif()
@@ -28,6 +28,8 @@ endmacro()
 
 RuntimeLibrary_check(default-C MultiThreadedDebugDLL)
 RuntimeLibrary_check(default-CXX MultiThreadedDebugDLL)
+RuntimeLibrary_check(empty-C "")
+RuntimeLibrary_check(empty-CXX "")
 RuntimeLibrary_check(MTd-C MultiThreadedDebug)
 RuntimeLibrary_check(MTd-CXX MultiThreadedDebug)
 RuntimeLibrary_check(MT-C MultiThreaded)

Tests/RunCMake/VS10Project/RuntimeLibrary.cmake

@@ -6,6 +6,10 @@ enable_language(CXX)
 add_library(default-C empty.c)
 add_library(default-CXX empty.cxx)
 
+set(CMAKE_MSVC_RUNTIME_LIBRARY "")
+add_library(empty-C empty.c)
+add_library(empty-CXX empty.cxx)
+
 set(CMAKE_MSVC_RUNTIME_LIBRARY "MultiThreadedDebug")
 add_library(MTd-C empty.c)
 add_library(MTd-CXX empty.cxx)