Merge topic 'cxxmodules-cmp0155-graceful-fallback-without-scanner'

1f507580a1 cmGlobalGenerator: give context about module queries
889aa0354a CMP0155: ignore scanning for sources if no scanner is available

Acked-by: Kitware Robot <kwrobot@kitware.com>
Tested-by: buildbot <buildbot@kitware.com>
Merge-request: !8925

Source/cmGeneratorTarget.cxx

@@ -9259,7 +9259,8 @@ void cmGeneratorTarget::CheckCxxModuleStatus(std::string const& config) const
 
   // If the generator doesn't support modules at all, error that we have
   // sources that require the support.
-  if (!this->GetGlobalGenerator()->CheckCxxModuleSupport()) {
+  if (!this->GetGlobalGenerator()->CheckCxxModuleSupport(
+        cmGlobalGenerator::CxxModuleSupportQuery::Expected)) {
     this->Makefile->IssueMessage(
       MessageType::FATAL_ERROR,
       cmStrCat(
@@ -9317,7 +9318,8 @@ bool cmGeneratorTarget::NeedCxxModuleSupport(std::string const& lang,
     return false;
   }
   return this->HaveCxxModuleSupport(config) == Cxx20SupportLevel::Supported &&
-    this->GetGlobalGenerator()->CheckCxxModuleSupport();
+    this->GetGlobalGenerator()->CheckCxxModuleSupport(
+      cmGlobalGenerator::CxxModuleSupportQuery::Inspect);
 }
 
 bool cmGeneratorTarget::NeedDyndep(std::string const& lang,
@@ -9361,14 +9363,20 @@ bool cmGeneratorTarget::NeedDyndepForSource(std::string const& lang,
     return true;
   }
 
+  bool haveRule = false;
   switch (this->HaveCxxModuleSupport(config)) {
     case Cxx20SupportLevel::MissingCxx:
     case Cxx20SupportLevel::NoCxx20:
       return false;
     case Cxx20SupportLevel::MissingRule:
+      break;
     case Cxx20SupportLevel::Supported:
+      haveRule = true;
       break;
   }
+  bool haveGeneratorSupport =
+    this->GetGlobalGenerator()->CheckCxxModuleSupport(
+      cmGlobalGenerator::CxxModuleSupportQuery::Inspect);
   auto const sfProp = sf->GetProperty("CXX_SCAN_FOR_MODULES");
   if (sfProp.IsSet()) {
     return sfProp.IsOn();
@@ -9388,8 +9396,9 @@ bool cmGeneratorTarget::NeedDyndepForSource(std::string const& lang,
     case cmPolicies::REQUIRED_ALWAYS:
     case cmPolicies::REQUIRED_IF_USED:
     case cmPolicies::NEW:
-      // The NEW behavior is to scan the source.
-      policyAnswer = true;
+      // The NEW behavior is to scan the source if the compiler supports
+      // scanning and the generator supports it.
+      policyAnswer = haveRule && haveGeneratorSupport;
       break;
   }
   return policyAnswer;

Source/cmGlobalGenerator.h

@@ -156,7 +156,17 @@ public:
 
   virtual bool InspectConfigTypeVariables() { return true; }
 
-  virtual bool CheckCxxModuleSupport() { return false; }
+  enum class CxxModuleSupportQuery
+  {
+    // Support is expected at the call site.
+    Expected,
+    // The call site is querying for support and handles problems by itself.
+    Inspect,
+  };
+  virtual bool CheckCxxModuleSupport(CxxModuleSupportQuery /*query*/)
+  {
+    return false;
+  }
 
   virtual bool IsGNUMakeJobServerAware() const { return false; }
 

Source/cmGlobalNinjaGenerator.cxx

@@ -882,13 +882,14 @@ bool cmGlobalNinjaGenerator::CheckLanguages(
   return true;
 }
 
-bool cmGlobalNinjaGenerator::CheckCxxModuleSupport()
+bool cmGlobalNinjaGenerator::CheckCxxModuleSupport(CxxModuleSupportQuery query)
 {
   if (this->NinjaSupportsDyndepsCxx) {
     return true;
   }
   bool const diagnose = !this->DiagnosedCxxModuleNinjaSupport &&
-    !this->CMakeInstance->GetIsInTryCompile();
+    !this->CMakeInstance->GetIsInTryCompile() &&
+    query == CxxModuleSupportQuery::Expected;
   if (diagnose) {
     std::ostringstream e;
     /* clang-format off */

Source/cmGlobalNinjaGenerator.h

@@ -475,7 +475,7 @@ public:
 
   bool IsSingleConfigUtility(cmGeneratorTarget const* target) const;
 
-  bool CheckCxxModuleSupport() override;
+  bool CheckCxxModuleSupport(CxxModuleSupportQuery query) override;
 
 protected:
   void Generate() override;

Source/cmGlobalVisualStudioVersionedGenerator.h

@@ -48,7 +48,7 @@ public:
 
   const char* GetAndroidApplicationTypeRevision() const override;
 
-  bool CheckCxxModuleSupport() override
+  bool CheckCxxModuleSupport(CxxModuleSupportQuery /*query*/) override
   {
     return this->SupportsCxxModuleDyndep();
   }

Source/cmPolicies.h

@@ -470,10 +470,10 @@ class cmMakefile;
     POLICY, CMP0154,                                                          \
     "Generated files are private by default in targets using file sets.", 3,  \
     28, 0, cmPolicies::WARN)                                                  \
-  SELECT(                                                                     \
-    POLICY, CMP0155,                                                          \
-    "C++ sources in targets with at least C++20 are scanned for imports", 3,  \
-    28, 0, cmPolicies::WARN)
+  SELECT(POLICY, CMP0155,                                                     \
+         "C++ sources in targets with at least C++20 are scanned for "        \
+         "imports when supported.",                                           \
+         3, 28, 0, cmPolicies::WARN)
 
 #define CM_SELECT_ID(F, A1, A2, A3, A4, A5, A6) F(A1)
 #define CM_FOR_EACH_POLICY_ID(POLICY)                                         \