cmGeneratorExpressionNode: implement `COMPILE_ONLY` genex

This generator expression is the inverse of `LINK_ONLY` and only coveys
usage requirements for the purposes of compilation. Its intended use is
to avoid needing to export targets that do not have link usage
requirements (e.g., header-only libraries) when used by another target.

See: #15415

4 files changed, 18 insertions, 6 deletions

diff --git a/Source/cmGeneratorExpressionDAGChecker.cxx b/Source/cmGeneratorExpressionDAGChecker.cxx
index 82a6c57..d51dbd0 100644
--- a/Source/cmGeneratorExpressionDAGChecker.cxx
+++ b/Source/cmGeneratorExpressionDAGChecker.cxx
@@ -27,6 +27,7 @@ cmGeneratorExpressionDAGChecker::cmGeneratorExpressionDAGChecker(
   , Content(content)
   , Backtrace(std::move(backtrace))
   , TransitivePropertiesOnly(false)
+  , CMP0131(false)
 {
   this->Initialize();
 }
@@ -41,6 +42,7 @@ cmGeneratorExpressionDAGChecker::cmGeneratorExpressionDAGChecker(
   , Content(content)
   , Backtrace()
   , TransitivePropertiesOnly(false)
+  , CMP0131(false)
 {
   this->Initialize();
 }
@@ -143,6 +145,12 @@ bool cmGeneratorExpressionDAGChecker::GetTransitivePropertiesOnly() const
   return this->Top()->TransitivePropertiesOnly;
 }
 
+bool cmGeneratorExpressionDAGChecker::GetTransitivePropertiesOnlyCMP0131()
+  const
+{
+  return this->Top()->CMP0131;
+}
+
 bool cmGeneratorExpressionDAGChecker::EvaluatingGenexExpression() const
 {
   return cmHasLiteralPrefix(this->Property, "TARGET_GENEX_EVAL:") ||
diff --git a/Source/cmGeneratorExpressionDAGChecker.h b/Source/cmGeneratorExpressionDAGChecker.h
index df1e005..1919b01 100644
--- a/Source/cmGeneratorExpressionDAGChecker.h
+++ b/Source/cmGeneratorExpressionDAGChecker.h
@@ -90,6 +90,9 @@ struct cmGeneratorExpressionDAGChecker
   bool GetTransitivePropertiesOnly() const;
   void SetTransitivePropertiesOnly() { this->TransitivePropertiesOnly = true; }
 
+  bool GetTransitivePropertiesOnlyCMP0131() const;
+  void SetTransitivePropertiesOnlyCMP0131() { this->CMP0131 = true; }
+
   cmGeneratorExpressionDAGChecker const* Top() const;
   cmGeneratorTarget const* TopTarget() const;
 
@@ -105,4 +108,5 @@ private:
   const cmListFileBacktrace Backtrace;
   Result CheckResult;
   bool TransitivePropertiesOnly;
+  bool CMP0131;
 };
diff --git a/Source/cmGeneratorExpressionNode.cxx b/Source/cmGeneratorExpressionNode.cxx
index a221498..ca61f75 100644
--- a/Source/cmGeneratorExpressionNode.cxx
+++ b/Source/cmGeneratorExpressionNode.cxx
@@ -1363,14 +1363,13 @@ static const struct CompileOnlyNode : public cmGeneratorExpressionNode
   {
     if (!dagChecker) {
       reportError(context, content->GetOriginalExpression(),
-                  "$<COMPILE_ONLY:...> may only be used via linking");
+                  "$<COMPILE_ONLY:...> may only be used for linking");
       return std::string();
     }
-    // Linking checks for the inverse, so compiling is the opposite.
     if (dagChecker->GetTransitivePropertiesOnly()) {
       return parameters.front();
     }
-    return std::string();
+    return std::string{};
   }
 } compileOnlyNode;
 
@@ -1389,8 +1388,7 @@ static const struct LinkOnlyNode : public cmGeneratorExpressionNode
                   "$<LINK_ONLY:...> may only be used for linking");
       return std::string();
     }
-    // Compile-only checks for the inverse, so linking is the opposite.
-    if (!dagChecker->GetTransitivePropertiesOnly()) {
+    if (!dagChecker->GetTransitivePropertiesOnlyCMP0131()) {
       return parameters.front();
     }
     return std::string();
diff --git a/Source/cmGeneratorTarget.cxx b/Source/cmGeneratorTarget.cxx
index 90cddb5..7e5ef0a 100644
--- a/Source/cmGeneratorTarget.cxx
+++ b/Source/cmGeneratorTarget.cxx
@@ -6791,6 +6791,7 @@ void cmGeneratorTarget::ExpandLinkItems(
   // requirements.
   if (interfaceFor == LinkInterfaceFor::Usage) {
     dagChecker.SetTransitivePropertiesOnly();
+    dagChecker.SetTransitivePropertiesOnlyCMP0131();
   }
   cmMakefile const* mf = this->LocalGenerator->GetMakefile();
   LookupLinkItemScope scope{ this->LocalGenerator };
@@ -8229,6 +8230,7 @@ void cmGeneratorTarget::ComputeLinkImplementationLibraries(
     // The $<LINK_ONLY> expression may be used to specify link dependencies
     // that are otherwise excluded from usage requirements.
     if (implFor == LinkInterfaceFor::Usage) {
+      dagChecker.SetTransitivePropertiesOnly();
       switch (this->GetPolicyStatusCMP0131()) {
         case cmPolicies::WARN:
         case cmPolicies::OLD:
@@ -8236,7 +8238,7 @@ void cmGeneratorTarget::ComputeLinkImplementationLibraries(
         case cmPolicies::REQUIRED_IF_USED:
         case cmPolicies::REQUIRED_ALWAYS:
         case cmPolicies::NEW:
-          dagChecker.SetTransitivePropertiesOnly();
+          dagChecker.SetTransitivePropertiesOnlyCMP0131();
           break;
       }
     }