Make cycles in target properties ignored, not an error.

Constructs such as these are an error as they are direct self-references:

 set_property(TARGET foo APPEND PROPERTY
   INCLUDE_DIRECTORIES $<TARGET_PROPERTY:foo,INCLUDE_DIRECTORIES>)
 set_property(TARGET foo APPEND PROPERTY
   INCLUDE_DIRECTORIES $<TARGET_PROPERTY:INCLUDE_DIRECTORIES>)

However, this is an indirect self-reference in a cycle, and not an error:

 set_property(TARGET foo APPEND PROPERTY
   INCLUDE_DIRECTORIES $<TARGET_PROPERTY:bar,INCLUDE_DIRECTORIES>)
 set_property(TARGET bar APPEND PROPERTY
   INCLUDE_DIRECTORIES $<TARGET_PROPERTY:foo,INCLUDE_DIRECTORIES>)

1 files changed, 9 insertions, 7 deletions

diff --git a/Source/cmGeneratorExpressionDAGChecker.cxx b/Source/cmGeneratorExpressionDAGChecker.cxx
index 4a9fc28..2e5b5ae 100644
--- a/Source/cmGeneratorExpressionDAGChecker.cxx
+++ b/Source/cmGeneratorExpressionDAGChecker.cxx
@@ -24,13 +24,14 @@ cmGeneratorExpressionDAGChecker::cmGeneratorExpressionDAGChecker(
   : Parent(parent), Target(target), Property(property),
     Content(content), Backtrace(backtrace)
 {
-  this->IsDAG = this->isDAG();
+  this->CheckResult = this->checkGraph();
 }
 
 //----------------------------------------------------------------------------
-bool cmGeneratorExpressionDAGChecker::check() const
+cmGeneratorExpressionDAGChecker::Result
+cmGeneratorExpressionDAGChecker::check() const
 {
-  return this->IsDAG;
+  return this->CheckResult;
 }
 
 //----------------------------------------------------------------------------
@@ -38,7 +39,7 @@ void cmGeneratorExpressionDAGChecker::reportError(
                   cmGeneratorExpressionContext *context,
                   const std::string &expr)
 {
-  if (this->IsDAG)
+  if (this->CheckResult == DAG)
     {
     return;
     }
@@ -91,16 +92,17 @@ void cmGeneratorExpressionDAGChecker::reportError(
 }
 
 //----------------------------------------------------------------------------
-bool cmGeneratorExpressionDAGChecker::isDAG() const
+cmGeneratorExpressionDAGChecker::Result
+cmGeneratorExpressionDAGChecker::checkGraph() const
 {
   const cmGeneratorExpressionDAGChecker *parent = this->Parent;
   while (parent)
     {
     if (this->Target == parent->Target && this->Property == parent->Property)
       {
-      return false;
+      return parent->Parent ? CYCLIC_REFERENCE : SELF_REFERENCE;
       }
     parent = parent->Parent;
     }
-  return true;
+  return DAG;
 }