Genex: Disallow LINKER_LANGUAGE only when used on a static library.

For shared libraries and executables, the linker_language is
indepenedent of the linked libraries.

Source/cmGeneratorExpressionEvaluator.cxx

@@ -790,11 +790,12 @@ static const struct TargetPropertyNode : public cmGeneratorExpressionNode
 
     if (propertyName == "LINKER_LANGUAGE")
       {
-      if (dagCheckerParent && dagCheckerParent->EvaluatingLinkLibraries())
+      if (target->LinkLanguagePropagatesToDependents() &&
+          dagCheckerParent && dagCheckerParent->EvaluatingLinkLibraries())
         {
         reportError(context, content->GetOriginalExpression(),
             "LINKER_LANGUAGE target property can not be used while evaluating "
-            "link libraries");
+            "link libraries for a static library");
         return std::string();
         }
       const char *lang = target->GetLinkerLanguage(context->Config);

Source/cmTarget.cxx

@@ -6241,7 +6241,7 @@ void cmTarget::ComputeImportInfo(std::string const& desired_config,
   }
 
   // Get the link languages.
-  if(this->GetType() == cmTarget::STATIC_LIBRARY)
+  if(this->LinkLanguagePropagatesToDependents())
     {
     std::string linkProp = "IMPORTED_LINK_INTERFACE_LANGUAGES";
     linkProp += suffix;
@@ -6470,7 +6470,7 @@ bool cmTarget::ComputeLinkInterface(const char* config, LinkInterface& iface,
       else
         {
         iface.Libraries = impl->Libraries;
-        if(this->GetType() == cmTarget::STATIC_LIBRARY)
+        if(this->LinkLanguagePropagatesToDependents())
           {
           // Targets using this archive need its language runtime libraries.
           iface.Languages = impl->Languages;
@@ -6539,7 +6539,7 @@ bool cmTarget::ComputeLinkInterface(const char* config, LinkInterface& iface,
             }
           }
         }
-      if(this->GetType() == cmTarget::STATIC_LIBRARY)
+      if(this->LinkLanguagePropagatesToDependents())
         {
         // Targets using this archive need its language runtime libraries.
         iface.Languages = impl->Languages;
@@ -6558,7 +6558,7 @@ bool cmTarget::ComputeLinkInterface(const char* config, LinkInterface& iface,
     iface.ImplementationIsInterface = true;
     iface.Libraries = impl->Libraries;
     iface.WrongConfigLibraries = impl->WrongConfigLibraries;
-    if(this->GetType() == cmTarget::STATIC_LIBRARY)
+    if(this->LinkLanguagePropagatesToDependents())
       {
       // Targets using this archive need its language runtime libraries.
       iface.Languages = impl->Languages;

Source/cmTarget.h

@@ -549,6 +549,9 @@ public:
 
   void FinalizeSystemIncludeDirectories();
 
+  bool LinkLanguagePropagatesToDependents() const
+  { return this->TargetTypeValue == STATIC_LIBRARY; }
+
 private:
   // The set of include directories that are marked as system include
   // directories.

Tests/RunCMake/Languages/LINK_LANGUAGE-genex.cmake

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