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>)

3 files changed, 25 insertions, 11 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;
 }
diff --git a/Source/cmGeneratorExpressionDAGChecker.h b/Source/cmGeneratorExpressionDAGChecker.h
index ffc84f8..48f26ed 100644
--- a/Source/cmGeneratorExpressionDAGChecker.h
+++ b/Source/cmGeneratorExpressionDAGChecker.h
@@ -25,12 +25,18 @@ struct cmGeneratorExpressionDAGChecker
                                   const GeneratorExpressionContent *content,
                                   cmGeneratorExpressionDAGChecker *parent);
 
-  bool check() const;
+  enum Result {
+    DAG,
+    SELF_REFERENCE,
+    CYCLIC_REFERENCE
+  };
+
+  Result check() const;
 
   void reportError(cmGeneratorExpressionContext *context,
                    const std::string &expr);
 private:
-  bool isDAG() const;
+  Result checkGraph() const;
 
 private:
   const cmGeneratorExpressionDAGChecker * const Parent;
@@ -38,7 +44,7 @@ private:
   const std::string Property;
   const GeneratorExpressionContent * const Content;
   const cmListFileBacktrace Backtrace;
-  bool IsDAG;
+  Result CheckResult;
 };
 
 #endif
diff --git a/Source/cmGeneratorExpressionEvaluator.cxx b/Source/cmGeneratorExpressionEvaluator.cxx
index 8b8b860..4b44ebe 100644
--- a/Source/cmGeneratorExpressionEvaluator.cxx
+++ b/Source/cmGeneratorExpressionEvaluator.cxx
@@ -381,10 +381,16 @@ static const struct TargetPropertyNode : public cmGeneratorExpressionNode
                                                content,
                                                dagCheckerParent);
 
-    if (!dagChecker.check())
+    switch (dagChecker.check())
       {
+    case cmGeneratorExpressionDAGChecker::SELF_REFERENCE:
       dagChecker.reportError(context, content->GetOriginalExpression());
       return std::string();
+    case cmGeneratorExpressionDAGChecker::CYCLIC_REFERENCE:
+      // No error. We just skip cyclic references.
+      return std::string();
+    case cmGeneratorExpressionDAGChecker::DAG:
+      break;
       }
 
     const char *prop = target->GetProperty(propertyName.c_str());