Genex: Make LINK_LANGUAGE report an error when evaluating link libraries.

Source/cmGeneratorExpressionEvaluator.cxx

@@ -415,8 +415,15 @@ static const struct LinkLanguageNode : public cmGeneratorExpressionNode
   std::string Evaluate(const std::vector<std::string> &parameters,
                        cmGeneratorExpressionContext *context,
                        const GeneratorExpressionContent *content,
-                       cmGeneratorExpressionDAGChecker *) const
+                       cmGeneratorExpressionDAGChecker *dagChecker) const
   {
+    if (dagChecker && dagChecker->EvaluatingLinkLibraries())
+      {
+      reportError(context, content->GetOriginalExpression(),
+          "$<LINK_LANGUAGE> expression can not be used while evaluating "
+          "link libraries");
+      return std::string();
+      }
     if (parameters.size() != 0 && parameters.size() != 1)
       {
       reportError(context, content->GetOriginalExpression(),

Tests/RunCMake/Languages/LINK_LANGUAGE-genex-result.txt

@@ -0,0 +1 @@
+1

Tests/RunCMake/Languages/LINK_LANGUAGE-genex-stderr.txt

@@ -0,0 +1,6 @@
+CMake Error:
+  Error evaluating generator expression:
+
+    \$<LINK_LANGUAGE>
+
+  \$<LINK_LANGUAGE> expression can not be used while evaluating link libraries

Tests/RunCMake/Languages/LINK_LANGUAGE-genex.cmake

@@ -0,0 +1,4 @@
+
+add_library(foo SHARED empty.cpp)
+add_library(bar SHARED empty.cpp)
+target_link_libraries(foo $<$<STREQUAL:$<LINK_LANGUAGE>,anything>:bar>)

Tests/RunCMake/Languages/RunCMakeTest.cmake

@@ -1,3 +1,4 @@
 include(RunCMake)
 
 run_cmake(NoLangSHARED)
+run_cmake(LINK_LANGUAGE-genex)

Tests/RunCMake/Languages/empty.cpp

@@ -0,0 +1,7 @@
+#ifdef _WIN32
+__declspec(dllexport)
+#endif
+int empty(void)
+{
+  return 0;
+}