cxxmodules: Fix CMP0155 NEW behavior when C++ compile features are not known

With CMP0155 NEW behavior, we scan C++ sources in targets using C++ 20,
i.e., in which the `cxx_std_20` feature is available.  However, our
check for C++ feature availability may incorrectly succeed in two cases:

* MSVC versions from VS 2013 do not model C++ standard levels,
  so we assume all features are available as a heuristic to let
  projects at least try compiling with them.

* During ABI detection the `CMAKE_CXX20_COMPILE_FEATURES` variable is not
  populated so we assume all features are available, knowing that our
  ABI detection project does not need them.

For purposes of detecting targets using C++ 20, we do not want to assume
`cxx_std_20` is available, so verify that we really know it is.

1 files changed, 18 insertions, 11 deletions

diff --git a/Source/cmGeneratorTarget.cxx b/Source/cmGeneratorTarget.cxx
index 96fcf7e..422d927 100644
--- a/Source/cmGeneratorTarget.cxx
+++ b/Source/cmGeneratorTarget.cxx
@@ -9093,20 +9093,27 @@ cmGeneratorTarget::Cxx20SupportLevel cmGeneratorTarget::HaveCxxModuleSupport(
   if (!state->GetLanguageEnabled("CXX")) {
     return Cxx20SupportLevel::MissingCxx;
   }
+
   cmValue standardDefault =
-    this->Target->GetMakefile()->GetDefinition("CMAKE_CXX_STANDARD_DEFAULT");
-  if (standardDefault && !standardDefault->empty()) {
-    cmStandardLevelResolver standardResolver(this->Makefile);
-    if (!standardResolver.HaveStandardAvailable(this, "CXX", config,
-                                                "cxx_std_20")) {
-      return Cxx20SupportLevel::NoCxx20;
-    }
+    this->Makefile->GetDefinition("CMAKE_CXX_STANDARD_DEFAULT");
+  if (!standardDefault || standardDefault->empty()) {
+    // We do not know any meaningful C++ standard levels for this compiler.
+    return Cxx20SupportLevel::NoCxx20;
+  }
+
+  cmStandardLevelResolver standardResolver(this->Makefile);
+  if (!standardResolver.HaveStandardAvailable(this, "CXX", config,
+                                              "cxx_std_20") ||
+      // During the ABI detection step we do not know the compiler's features.
+      // HaveStandardAvailable may return true as a fallback, but in this code
+      // path we do not want to assume C++ 20 is available.
+      this->Makefile->GetDefinition("CMAKE_CXX20_COMPILE_FEATURES")
+        .IsEmpty()) {
+    return Cxx20SupportLevel::NoCxx20;
   }
-  // Else, an empty CMAKE_CXX_STANDARD_DEFAULT means CMake does not detect and
-  // set a default standard level for this compiler, so assume all standards
-  // are available.
+
   cmValue scandepRule =
-    this->Target->GetMakefile()->GetDefinition("CMAKE_CXX_SCANDEP_SOURCE");
+    this->Makefile->GetDefinition("CMAKE_CXX_SCANDEP_SOURCE");
   if (!scandepRule) {
     return Cxx20SupportLevel::MissingRule;
   }