CSharp: Fix regression in VS project type selection for custom target

A target created by `add_custom_target` should always be a `.vcxproj`
file even if it has `.cs` sources involved in custom commands and such.
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) does not check the target type
and so is not a suitable check for deciding the project file extension.

The `HasLanguage` method was an attempt at an abstraction that turns
out not to work very well.  Replace it with a dedicated `IsCSharpOnly`
method that considers the target type, sources, and non-transitive
`LINKER_LANGUAGE`.

Fixes: #18515

1 files changed, 1 insertions, 5 deletions

diff --git a/Source/cmGeneratorTarget.h b/Source/cmGeneratorTarget.h
index b1daa53..5ed8e5a 100644
--- a/Source/cmGeneratorTarget.h
+++ b/Source/cmGeneratorTarget.h
@@ -372,11 +372,7 @@ public:
   void GetLanguages(std::set<std::string>& languages,
                     std::string const& config) const;
 
-  // Evaluate if the target uses the given language for compilation
-  // and/or linking. If 'exclusive' is true, 'language' is expected
-  // to be the only language used in source files for the target.
-  bool HasLanguage(std::string const& language, std::string const& config,
-                   bool exclusive = true) const;
+  bool IsCSharpOnly() const;
 
   void GetObjectLibrariesCMP0026(
     std::vector<cmGeneratorTarget*>& objlibs) const;