CSharp: Fix regression in VS project type selection

A that target contains only `.cs` sources should be generated as a
`.csproj` project even if it links to non-CSharp static libraries.
The latter case was broken by refactoring in commit v3.12.0-rc1~160^2~7
(remove TargetIsCSharpOnly() and use methods from cmGeneratorTarget,
2018-03-19).  The reason is that the `HasLanguage` method added by
commit v3.12.0-rc1~239^2~6 (cmGeneratorTarget: add HasLanguage() as
wrapper for GetLanguages(), 2018-03-19) enforces its "exclusive" check
on the combined set of source file languages and the link language.
To restore the original `TargetIsCSharpOnly` semantics, update
`HasLanguage` to enforce exclusiveness only on the list of sources.

Fixes: #18239

2 files changed, 9 insertions, 0 deletions

diff --git a/Tests/CSharpLinkToCxx/CMakeLists.txt b/Tests/CSharpLinkToCxx/CMakeLists.txt
index 153c57c..a3067af 100644
--- a/Tests/CSharpLinkToCxx/CMakeLists.txt
+++ b/Tests/CSharpLinkToCxx/CMakeLists.txt
@@ -21,3 +21,9 @@ target_link_libraries(CSharpLinkToCxx CLIApp)
 # because it is unmanaged
 add_library(CppNativeApp SHARED cpp_native.hpp cpp_native.cpp)
 target_link_libraries(CSharpLinkToCxx CppNativeApp)
+
+# Link a static C++ library into the CSharp executable.
+# We do not actually use any symbols but this helps cover
+# link language selection.
+add_library(CppStaticLib STATIC cpp_static.cpp)
+target_link_libraries(CSharpLinkToCxx CppStaticLib)
diff --git a/Tests/CSharpLinkToCxx/cpp_static.cpp b/Tests/CSharpLinkToCxx/cpp_static.cpp
new file mode 100644
index 0000000..9af2b6e
--- /dev/null
+++ b/Tests/CSharpLinkToCxx/cpp_static.cpp
@@ -0,0 +1,3 @@
+void cpp_static()
+{
+}