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

Source/cmExportFileGenerator.cxx

@@ -788,7 +788,7 @@ void cmExportFileGenerator::SetImportDetailProperties(
     std::string propval;
     if (auto* p = target->GetProperty("COMMON_LANGUAGE_RUNTIME")) {
       propval = p;
-    } else if (target->HasLanguage("CSharp", config)) {
+    } else if (target->IsCSharpOnly()) {
       // C# projects do not have the /clr flag, so we set the property
       // here to mark the target as (only) managed (i.e. no .lib file
       // to link to). Otherwise the  COMMON_LANGUAGE_RUNTIME target

Source/cmGeneratorTarget.cxx

@@ -5221,20 +5221,23 @@ void cmGeneratorTarget::GetLanguages(std::set<std::string>& languages,
   }
 }
 
-bool cmGeneratorTarget::HasLanguage(std::string const& language,
-                                    std::string const& config,
-                                    bool exclusive) const
+bool cmGeneratorTarget::IsCSharpOnly() const
 {
-  std::set<std::string> languages;
-  this->GetLanguages(languages, config);
-  // The "exclusive" check applies only to source files and not
-  // the linker language which may be affected by dependencies.
-  if (exclusive && languages.size() > 1) {
+  // Only certain target types may compile CSharp.
+  if (this->GetType() != cmStateEnums::SHARED_LIBRARY &&
+      this->GetType() != cmStateEnums::STATIC_LIBRARY &&
+      this->GetType() != cmStateEnums::EXECUTABLE) {
     return false;
   }
-  // add linker language (if it is different from compiler languages)
-  languages.insert(this->GetLinkerLanguage(config));
-  return languages.count(language) > 0;
+  std::set<std::string> languages;
+  this->GetLanguages(languages, "");
+  // Consider an explicit linker language property, but *not* the
+  // computed linker language that may depend on linked targets.
+  const char* linkLang = this->GetProperty("LINKER_LANGUAGE");
+  if (linkLang && *linkLang) {
+    languages.insert(linkLang);
+  }
+  return languages.size() == 1 && languages.count("CSharp") > 0;
 }
 
 void cmGeneratorTarget::ComputeLinkImplementationLanguages(
@@ -5555,6 +5558,5 @@ cmGeneratorTarget::ManagedType cmGeneratorTarget::GetManagedType(
   // C# targets are always managed. This language specific check
   // is added to avoid that the COMMON_LANGUAGE_RUNTIME target property
   // has to be set manually for C# targets.
-  return this->HasLanguage("CSharp", config) ? ManagedType::Managed
-                                             : ManagedType::Native;
+  return this->IsCSharpOnly() ? ManagedType::Managed : ManagedType::Native;
 }

Source/cmGeneratorTarget.h

@@ -364,11 +364,7 @@ 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;

Source/cmGlobalVisualStudio71Generator.cxx

@@ -98,7 +98,7 @@ void cmGlobalVisualStudio71Generator::WriteProject(std::ostream& fout,
     ext = ".vfproj";
     project = "Project(\"{6989167D-11E4-40FE-8C1A-2192A86A7E90}\") = \"";
   }
-  if (t->HasLanguage("CSharp", "")) {
+  if (t->IsCSharpOnly()) {
     ext = ".csproj";
     project = "Project(\"{FAE04EC0-301F-11D3-BF4B-00C04F79EFBC}\") = \"";
   }

Source/cmVisualStudio10TargetGenerator.cxx

@@ -215,12 +215,11 @@ static bool cmVS10IsTargetsFile(std::string const& path)
   return cmSystemTools::Strucmp(ext.c_str(), ".targets") == 0;
 }
 
-static std::string computeProjectFileExtension(cmGeneratorTarget const* t,
-                                               const std::string& config)
+static std::string computeProjectFileExtension(cmGeneratorTarget const* t)
 {
   std::string res;
   res = ".vcxproj";
-  if (t->HasLanguage("CSharp", config)) {
+  if (t->IsCSharpOnly()) {
     res = ".csproj";
   }
   return res;
@@ -316,8 +315,8 @@ void cmVisualStudio10TargetGenerator::Generate()
       this->GeneratorTarget->GetProperty("EXTERNAL_MSPROJECT")) {
     return;
   }
-  const std::string ProjectFileExtension = computeProjectFileExtension(
-    this->GeneratorTarget, *this->Configurations.begin());
+  const std::string ProjectFileExtension =
+    computeProjectFileExtension(this->GeneratorTarget);
   if (ProjectFileExtension == ".vcxproj") {
     this->ProjectType = vcxproj;
     this->Managed = false;
@@ -1399,8 +1398,7 @@ void cmVisualStudio10TargetGenerator::WriteGroups()
   std::string path = this->LocalGenerator->GetCurrentBinaryDirectory();
   path += "/";
   path += this->Name;
-  path += computeProjectFileExtension(this->GeneratorTarget,
-                                      *this->Configurations.begin());
+  path += computeProjectFileExtension(this->GeneratorTarget);
   path += ".filters";
   cmGeneratedFileStream fout(path.c_str());
   fout.SetCopyIfDifferent(true);
@@ -3734,7 +3732,7 @@ void cmVisualStudio10TargetGenerator::WriteProjectReferences(Elem& e0)
       path = lg->GetCurrentBinaryDirectory();
       path += "/";
       path += dt->GetName();
-      path += computeProjectFileExtension(dt, *this->Configurations.begin());
+      path += computeProjectFileExtension(dt);
     }
     ConvertToWindowsSlash(path);
     Elem e2(e1, "ProjectReference");
@@ -3761,7 +3759,7 @@ void cmVisualStudio10TargetGenerator::WriteProjectReferences(Elem& e0)
       // Workaround for static library C# targets
       if (referenceNotManaged &&
           dt->GetType() == cmStateEnums::STATIC_LIBRARY) {
-        referenceNotManaged = !dt->HasLanguage("CSharp", "");
+        referenceNotManaged = !dt->IsCSharpOnly();
       }
       if (referenceNotManaged) {
         e2.Element("ReferenceOutputAssembly", "false");

Tests/RunCMake/CSharpCustomCommand/RunCMakeTest.cmake

@@ -1,5 +1,13 @@
 include(RunCMake)
 
+function(run_TargetWithCommand)
+  set(RunCMake_TEST_BINARY_DIR ${RunCMake_BINARY_DIR}/TargetWithCommand-build)
+  run_cmake(TargetWithCommand)
+  set(RunCMake_TEST_NO_CLEAN 1)
+  run_cmake_command(TargetWithCommand-build ${CMAKE_COMMAND} --build . --config Debug)
+endfunction()
+run_TargetWithCommand()
+
 # Use a single build tree for a few tests without cleaning.
 set(RunCMake_TEST_BINARY_DIR ${RunCMake_BINARY_DIR}/CommandWithOutput-build)
 set(RunCMake_TEST_NO_CLEAN 1)

Tests/RunCMake/CSharpCustomCommand/TargetWithCommand-build-stdout.txt

@@ -0,0 +1 @@
+Custom target with CSharp source

Tests/RunCMake/CSharpCustomCommand/TargetWithCommand.cmake

@@ -0,0 +1,4 @@
+enable_language(CSharp)
+
+add_custom_target(drive ALL SOURCES dummy.cs
+  COMMAND ${CMAKE_COMMAND} -E echo "Custom target with CSharp source")